0x00000021 Fixes for QUOTA_UNDERFLOW on Windows

Stop code 0x00000021 can appear after a recent driver, software, or hardware change. A practical first response is to undo the newest change, then install pending Windows updates and retest. If the blue screen continues, use the sections below to narrow it down by when the crash happens and what changed just before it started.

This guide is part of our Windows BSOD stop codes: full list and fixes series.

Fast triage in 3 steps: first, undo the newest driver, utility, or device change; second, check whether the crash happens at startup, shutdown, or under load; third, if the pattern survives rollback and uninstall work, move to Windows repair commands and memory testing.

For broader context, see this BSOD troubleshooting hub and this driver troubleshooting hub.

What does stop code 0x00000021 mean?

Officially, QUOTA_UNDERFLOW has a bug check value of 0x00000021, and it indicates that quota charges have been mishandled by returning more quota to a particular block than was previously charged. (source)

Why this stop code usually points to kernel-mode software

Because the bug check refers to quota handling inside Windows, recent low-level changes deserve attention first. That includes newly installed or recently updated drivers, device-related software, and utilities that integrate closely with storage, networking, or security.

When the timing is clear, start with the newest change rather than making multiple changes at once.

Likely causes, ranked by odds

Use the first crash date and any recent system changes to decide where to start. Device drivers, newly added hardware, and recently installed software are the most direct items to review because they can be reversed with standard Windows tools.

1. Recent driver changes. Check recent storage, chipset, network, and GPU driver changes in Start > Device Manager.
2. Recently installed low-level software. If the BSOD began after installing security software, backup software, VPN software, encryption software, or virtualization software, test with a clean boot and remove the newest item if the system stabilizes.
3. Windows file corruption or hardware checks. If no recent change lines up with the first crash, open an elevated Command Prompt and run sfc /scannow; if it finds corruption or the BSOD still returns, run DISM /Online /Cleanup-Image /RestoreHealth; after that, schedule a memory test with mdsched.exe.

According to Bug Check 0x21 QUOTA_UNDERFLOW – Windows drivers — The bug check value for QUOTA_UNDERFLOW is 0x00000021. (source)

Did 0x00000021 start after a driver or device change?

If the first crash started right after a driver install, firmware utility, dock, GPU package, storage update, or newly attached device, reverse that change first. Device Manager and a simple disconnect test are the fastest standard checks.

Roll back or remove the newest driver

If the first crash followed a driver install or device-related software update, treat that change as the starting point until testing proves otherwise.

1. Open Start > Device Manager.
2. Expand Storage controllers, IDE ATA/ATAPI controllers, Network adapters, Display adapters, and System devices.
3. Right-click the recently changed device > Properties > Driver.
4. Select Roll Back Driver if it is available. If it is not, choose Uninstall Device, restart, and let Windows load the prior or inbox driver.
5. If the crash began after new hardware, shut down the PC, disconnect only that newly added device, start Windows again, and use the PC long enough to repeat the normal trigger before reconnecting it.

Apply updates in the right order

Windows Update is worth doing early, especially after a rollback or uninstall, so Windows can install any pending fixes.

1. On Windows 11, go to Settings > Windows Update. On Windows 10, go to Settings > Update & Security > Windows Update. (bleepingcomputer.com)
2. Click Check for updates, install every security, cumulative, and other critical Windows update offered, restart if prompted, then open the same page again and review Optional updates only for device drivers tied to the failing hardware.
3. Restart and retest startup, shutdown, and the workload that caused the BSOD.
4. If the vendor provides a newer stable driver for the same device, install it only after Windows finishes updating.

When Driver Verifier fits this branch

Driver Verifier is best reserved for cases where rollback, uninstall, and update work still leave a strong third-party driver pattern. Because it can make an unstable system crash more quickly, make sure Safe Mode is available first. If you need a walkthrough, use this Driver Verifier guide.

1. First make sure Safe Mode is reachable. If normal startup fails later, boot into Safe Mode and remove the suspect driver from Device Manager.
2. Use Driver Verifier only after removing the newest hardware and testing the inbox or rolled-back driver path.
3. If boot stability gets worse, return to Safe Mode, uninstall the flagged driver or use System Restore to return to the pre-test state.

Does this stop code usually happen after installing software or hardware?

It can. If the first blue screen appears right after a new utility or device is added, remove that exact change and retest before moving on to broader repair steps.

Why filter-style utilities are frequent suspects

Software that integrates closely with storage, networking, or security can add services or drivers that load with Windows. If the timing matches the start of the crashes, remove that software completely and test again.

Clean-boot and removal steps

A clean boot helps determine whether a third-party service is involved. If the system stops crashing in a clean boot state, remove the most recently installed software that matches the timing of the problem.

1. Press Win + R, type msconfig, and press Enter.
2. Open the Services tab, check Hide all Microsoft services, then click Disable all.
3. Restart and test the same action that triggered the BSOD.
4. If the crash stops, open Settings > Apps > Installed apps on Windows 11 or Settings > Apps on Windows 10, sort by install date, and uninstall the newest security, backup, VPN, encryption, virtualization, or similar low-level tool that was added before the blue screens began.
5. After uninstalling, restart, then test both a full startup and a full shutdown. If the system stays stable, leave that software uninstalled and check the vendor for an updated build before installing it again.

Tools worth checking first

Check software categories that commonly install drivers or background services and were installed or updated shortly before the first BSOD.

• Antivirus and endpoint security
• Backup and disk-imaging software
• VPN clients and network filters
• Encryption and drive-protection tools
• Virtualization software and virtual switches

Can outdated drivers cause a QUOTA_UNDERFLOW BSOD?

Yes. Outdated, corrupted, or newly changed drivers are worth checking with Device Manager, especially when the crash started after an update or hardware change.

Driver classes that deserve priority

Start with devices that were changed recently or are active during boot, shutdown, storage access, and network activity.

1. Open Start > Device Manager.
2. For laptops, review Network adapters, Storage controllers, and power-related entries under System devices, especially if the crashes started around sleep, resume, docking, or Wi‑Fi changes.
3. For systems showing 0x00000021 after update, inspect the drivers changed most recently, then use Roll Back Driver where available.
4. For systems showing 0x00000021 with a VPN, first confirm the pattern by running a clean boot; if the crashes stop, uninstall that VPN client from Settings > Apps, restart, and test again before reinstalling it.

Does 0x00000021 happen during startup, shutdown, or gaming?

Crash timing helps narrow the next step. Startup issues point to drivers that load early, shutdown issues point to software that stays active until power-off, and load-related crashes are a reason to review recent GPU changes and run memory checks.

Startup crashes

If the system crashes before or just after sign-in, Safe Mode is the quickest way to reduce third-party startup items and test whether Windows can stay stable with fewer drivers loaded.

1. Boot into Safe Mode.
2. Open Start > Device Manager and uninstall the most recent storage, network, GPU, or chipset driver change.
3. Restart normally. If Windows loads, install pending updates from Settings > Windows Update or Settings > Update & Security > Windows Update.
4. If the system remains unstable, use System Restore to return to the point before the first crash.

Shutdown crashes

If the blue screen appears during shutdown, focus on software that loads background services or drivers and was installed or updated shortly before the issue began.

1. Run a clean boot with msconfig.
2. Uninstall the newest antivirus, backup, VPN, encryption, or device-management utility.
3. Restart, then test a full shutdown from Start > Power > Shut down.
4. If the BSOD stops only after removal, leave that tool uninstalled and check for a newer vendor build before putting it back on the system.

Gaming or heavy-load crashes

If the crash appears only under heavy use, focus on recently changed graphics drivers and memory testing before moving on to broader hardware replacement.

1. Roll back or reinstall the GPU driver from Start > Device Manager > Display adapters.
2. Remove any recently installed gaming-related or hardware-management utility through Settings > Apps if the timing lines up with the first crash.
3. Run mdsched.exe and choose Restart now and check for problems.
4. If the system is a workstation doing large file jobs, temporarily uninstall backup software and virtualization software, restart, and repeat the same workload to see whether the crash returns without those components present.

How do I fix 0x00000021 on Windows 10 or 11?

Start by reversing the latest low-level change, updating Windows, and testing again. If that does not help, move through Device Manager, clean boot, SFC, DISM, memory testing, and storage checks one step at a time.

Diagnostic decision tree

Use the first clear trigger to decide the next move.

Symptom-to-fix matrix

Use this matrix when the machine is stable enough to test one change at a time.

Should I run SFC or DISM for a quota underflow blue screen?

Yes, but after checking recent drivers, utilities, and hardware changes first. SFC and DISM are repair tools for Windows files, not a substitute for reversing a recent change.

Run system file and image repairs

Standard Windows repair order is to run sfc /scannow first, then run DISM /Online /Cleanup-Image /RestoreHealth if needed. See this SFC and DISM repair guide for a full walkthrough.

1. Open Command Prompt as administrator.
2. Run sfc /scannow.
3. If SFC reports errors or the crashes continue, run DISM /Online /Cleanup-Image /RestoreHealth.
4. Restart and retest the exact trigger: startup, shutdown, gaming, or the workload that caused the BSOD.

Why these checks matter here

These checks help repair damaged Windows files that may keep the system unstable after another issue started. They are most useful after rollback, uninstall, and update steps have already been tried.

Can bad RAM or a disk problem trigger this stop code?

Yes. Memory and storage checks make sense if recent driver or software changes do not explain the timing, or if the crashes happen under heavy load or during large file activity.

Memory Diagnostics and storage checks

Windows Memory Diagnostic and disk checks are standard next steps once recent changes have been ruled out.

1. Press Win + R, type mdsched.exe, and press Enter.
2. Select Restart now and check for problems.
3. After Windows loads again, open Command Prompt as administrator and run chkdsk.
4. If the BSOD still returns, review the minidump for the faulting module or move to System Restore if the crashes began recently.

If that did not work, try this next

If rollback, uninstall, clean boot, repair commands, and memory checks do not help, move to recovery steps and preserve evidence for deeper review.

1. Boot into Safe Mode and confirm whether the system stays stable there.
2. Use System Restore to return to a point before the first crash.
3. If the machine still blue-screens after rollback, uninstall, clean boot, SFC, DISM, memory testing, and chkdsk, stop self-troubleshooting and review the dump file professionally or through the hardware or software vendor.

Prevention

Install low-level software one change at a time and test system stability after each change. That makes it much easier to identify the exact driver, utility, or device that started the problem.

Frequently asked questions

Is 0x00000021 the same as QUOTA_UNDERFLOW?

Yes. They refer to the same stop code.

When should I use Driver Verifier for this BSOD?

Use it after rollback, uninstall, Windows Update, and clean-boot testing still leave a pattern that points to a third-party driver. Have a recovery path first: Safe Mode access, System Restore available, and the suspect driver identified closely enough that you can remove it if boot stability gets worse.

How do I fix 0x00000021 on a laptop?

On a laptop, start with Wi‑Fi, Bluetooth, storage, GPU, and chipset drivers in Device Manager, then remove recent VPN, security, docking, or vendor utility changes made just before the first crash. Sleep, resume, and shutdown timing can help narrow which driver or utility to test first.

What if 0x00000021 started after a Windows update?

If it appeared right after updating Windows, inspect the drivers changed alongside that update, especially storage, network, and GPU entries in Device Manager. Apply all remaining Windows Update items, then use Roll Back Driver where available. If the crash persists, clean boot and remove any low-level utility installed around the same time.

Can a VPN cause 0x00000021?

It can. If a VPN was installed or updated shortly before the crashes started, clean boot first, then uninstall the VPN from Settings > Apps and retest before reinstalling a newer version.

Can bad RAM or a disk problem trigger this stop code?

Yes. Run mdsched.exe after software rollback and removal work is done, then run chkdsk if crashes also happen during heavy file activity, patching, or large backup jobs. For recovery paths, these Safe Mode steps and this System Restore guide are the pages to keep open.

Sources

• Microsoft Learn: Bug Check 0x21: QUOTA_UNDERFLOW
• Microsoft Support: Use the System File Checker tool to repair missing or corrupted system files
• Microsoft Learn: DISM operating system package-servicing command-line options
• Microsoft Support: Open Device Manager
• Microsoft Support: Windows Update: FAQ
• Microsoft Support: Windows Memory Diagnostic
• source
• bleepingcomputer.com
• forums.tomshardware.com