Windows 10 to Windows 11 upgrade partition requirements
Some PCs upgrading from Windows 11 23H2 or 24H2 to 25H2 run into a specific, confusing failure: "We couldn't update the system reserved partition," paired with error code 0x800f0922 (also logged as 0xc1900104). A Microsoft Q&A thread posted late last year reports a connection between this failure and devices running a 100 MB EFI System Partition, smaller than Microsoft's current sizing minimum (Microsoft Q&A). That's the scenario driving this look at Windows 10 to Windows 11 upgrade partition requirements: what the EFI partition needs to look like, and how to check yours before or after you upgrade.
That documented failure covers the later Windows 11 feature-update path specifically, not a confirmed wave of Windows 10-to-Windows 11 upgrade failures. Still, the same sizing minimums apply to any UEFI/GPT installation, which is why checking your ESP is worth doing whether you're planning a move off Windows 10 or already stuck mid-upgrade on Windows 11. This guide shows how to check your actual EFI partition size and free space, what a small or full partition does and doesn't prove, and what to do if you've already hit this exact error.
What's confirmed about this error, and what isn't
A response in that Q&A thread explains the error typically surfaces during Setup's SafeOS phase, when the installer tries to write new boot binaries and runs out of room on the EFI partition (Microsoft Q&A). That's a community explanation from a Q&A response, not a Microsoft support article, servicing advisory, or release-health notice; the supplied sources here don't include one, so treat it as plausible rather than official.
Separately, an independent administrator's write-up published last fall describes the same error appearing on HP laptops during Windows 11 upgrades pushed through SCCM or installation media. That report attributes the failure to HP firmware data nearly filling the default 100 MB EFI partition Windows 10 allocates (Orhan Çalışkan). That's one vendor-specific case study, not proof every HP machine, or every machine with a 100 MB ESP, hits the same wall.
The Q&A response also notes that Windows Update's failure detection is reactive rather than pre-flight: it flags trouble after the fact instead of warning you beforehand. That means the same generic error code can, in theory, point to other installer problems too. An undersized EFI partition is a real, plausible contributing factor on some systems. It isn't a confirmed root cause every time the message appears. Don't assume you have an ESP problem just because you've seen this error once; the rest of this guide shows how to check, rather than guess.
Windows 10 to Windows 11 partition layout: EFI, MSR, and recovery partitions
Part of the confusion comes from the error's own wording. "System reserved partition" is old terminology that doesn't name the EFI System Partition directly, and nothing in the supplied guidance treats the two as equivalent. A Microsoft Q&A guide for reinstalling Windows 11, posted last year, does use "System Reserved partition" informally to describe where recovery and diagnostic files live (Microsoft Q&A), but that's community phrasing in a support thread, not Microsoft's documented partition architecture. Don't read it as proof "System Reserved" equals the ESP.
On an actual UEFI/GPT installation, three separate partitions do three separate jobs, and confusing Windows 11 recovery and system reserved partitions with the EFI System Partition itself is the fastest way to end up editing the wrong one:
- EFI System Partition (ESP): formatted FAT32, minimum 200 MB on 512-byte/512e sector drives or 300 MB on 4K native sector drives. This is the specific partition implicated in the error covered here (Microsoft Learn).
- Microsoft Reserved Partition (MSR): exactly 16 MB, no drive letter, no user-facing files. It sits next to the ESP but isn't the same partition.
- Recovery partition: separate again, generally at least 300 MB, with the recovery image itself commonly landing between 500 and 700 MB depending on drivers and languages included (Microsoft Learn). Its size has nothing to do with how much room is left on the ESP.
Those ESP numbers are Microsoft's current minimums, updated earlier this year, for building or reimaging a UEFI/GPT drive today. They aren't a servicing advisory diagnosing this specific upgrade failure, and Microsoft's documentation doesn't claim that either. Treat them as the benchmark for a healthy partition table, not a guaranteed pass/fail line for every existing upgrade.
How to check your Windows 11 EFI system partition size
Before touching anything, confirm you're on a UEFI/GPT system rather than an older BIOS/MBR one. Press Windows+R, type msinfo32, and check the "BIOS Mode" field. None of the sizing figures above apply to a Legacy/BIOS/MBR drive, which uses a different boot partition model entirely, and Microsoft's deployment documentation treats the two as non-interchangeable (Microsoft Learn).
Open Command Prompt as an administrator and run:
diskpart
list disk
Match a listed capacity to your known drive size to confirm which disk is your main internal one; don't assume it's disk 0. Then select it and list its partitions:
select disk [your disk number]
list partition
These three commands only display information. The next step, assigning a drive letter, changes how Windows exposes the partition and needs to be reversed once you're done (Microsoft Learn).
Microsoft's deployment examples use S for the system partition, W for Windows, and R for recovery, while the MSR never gets a letter at all (Microsoft Learn). Treat that as a guide, not a guarantee: diskpart's list partition output identifies entries by number, type, size, and offset, not plain-language labels. If you can't confidently tell which one is the EFI System Partition, stop here rather than guess.
PowerShell offers a more direct route, filtering by the partition's GPT type ID instead of relying on how diskpart labels things onscreen. One independent troubleshooting script written for HP laptops identifies the ESP this way:
Get-Partition -DiskNumber 0 | Where-Object {$_.GptType -eq '{C12A7328-F81F-11D2-BA4B-00A0C93EC93B}'}
That returns the EFI System Partition specifically, by type, regardless of labeling elsewhere (Orhan Çalışkan).
Once you've confirmed the right partition, assign it a temporary letter:
select partition [your partition number]
assign letter=S
exit
Don't use X; Microsoft reserves that letter for Windows PE (Microsoft Learn). Check the partition's total size in File Explorer and compare it to the correct benchmark: 200 MB on 512-byte/512e sector drives, 300 MB on 4K native sector drives. If you can't determine which sector type your drive uses, don't use size alone to judge whether the partition is adequate; check your drive manufacturer's spec sheet first.
Free space is harder to read reliably than total size. The EFI partition won't show its contents through a normal folder browse, since Windows hides system files there by default, and none of the supplied guidance here covers a specific free-space inspection method beyond a dedicated partitioning tool. Treat total size as your main data point unless you're comfortable using a disk utility built for the job. A 100 MB partition that's mostly empty behaves differently from one nearly full of OEM firmware data, which is the exact condition described in the HP case study, though that report doesn't establish a general rule about how much free space counts as safe on other machines (Orhan Çalışkan).
When you're done looking, remove the temporary letter:
diskpart
select disk [your disk number]
select partition [your partition number]
remove letter=S
exit
What your findings mean
Don't resize or delete a partition solely because it's 100 MB. Here's how to read what you found:
- ESP meets or exceeds 200/300 MB with plenty of free space, no error yet: Continue troubleshooting or upgrading normally. The capacity theory is less likely to be your problem, though nothing here guarantees a smooth upgrade regardless of partition size.
- ESP is around 100 MB, no error yet: Worth treating as a risk factor based on the reported 25H2 scenario, not proof of a coming failure. Don't repartition, delete, or resize anything based on size alone.
- You've already seen "We couldn't update the system reserved partition" or 0x800f0922: Back up your personal files. Retry the update once. Check your update history to confirm the displayed code before investigating the ESP further, then check your PC manufacturer's support page for a firmware update specific to your model. Firmware behavior is directly implicated in the HP case study covered here (Orhan Çalışkan), so it's worth a check, not a guarantee of a fix.
If preliminary checks don't clear the error
A clean install is the next option, and it's a destructive one. Microsoft Q&A guidance for reinstalling Windows 11 describes deleting every partition on the target disk, starting from the bottom of the list, until only unallocated space remains, then letting Setup split that space into a new layout (Microsoft Q&A). That confirms Setup can rebuild a working partition table from a blank disk. It doesn't establish that a clean install is the best or most reliable fix for this exact error, only that it's a documented way to reset the layout. It also erases everything on the target disk, so try official troubleshooting or a manufacturer support call first, especially if you're not confident identifying disks and partitions on your own.
Before deleting anything, confirm you've picked the right drive. Compare each listed capacity to your known drive size rather than guessing, especially if your PC has more than one internal drive or a USB installer plugged in. Windows PE reassigns drive letters alphabetically after a reboot, and that assignment can shift depending on which USB devices are connected, so don't rely on an assumed letter like C: to identify your system disk (Microsoft Learn).
Two more fixes circulate in online troubleshooting threads, and neither belongs in a routine walkthrough. One Q&A response describes deleting the 16 MB Microsoft Reserved Partition to make room for a larger ESP, then recreating the MSR afterward; the supplied documentation doesn't offer this as a repair procedure, and a wrong step here can leave a drive that won't boot (Microsoft Q&A). A separate independent script deletes an approximately 40 MB file the report identifies at EFI\HP\DEVFW\firmware.bin on certain HP laptops; its developer calls it experimental and built specifically for HP hardware (Orhan Çalışkan). Do not attempt either method unless you're an experienced administrator with a verified full-disk image you can restore.
What to do next
Check your EFI partition's size before starting any Windows 10 to Windows 11 upgrade, especially on a PC that's a few years old and might still be carrying its original 100 MB layout. If you haven't seen the error, note what you found and leave the partition alone; a small ESP is a reason to watch for trouble, not a reason to start deleting or resizing today. If "We couldn't update the system reserved partition" or 0x800f0922 does show up, confirm the code, back up your files, check for a firmware update from your manufacturer, and treat MSR deletion, firmware-file removal, or a clean install as advanced recovery options rather than first moves. Contact the PC manufacturer or Microsoft support before modifying the ESP directly if you're not sure which step is safe for your setup.
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