Windows couldn't be installed. We couldn't update the system reserved partition is an error message that you may receive when you try to upgrade Windows.
I ran into the issue on a machine running Windows 10 Insider Builds but users experienced the issue in earlier versions of Windows as well. The error seemed to be common on Windows 8.1 systems that users tried to upgrade to Windows 10.
The system reserved partition contains boot information for Windows. Microsoft notes that users may receive the error "We couldn't update the system reserved partition" if it does not have enough free space. Users may receive error code 0xc1900104 or error code 0x800f0922 when that is the case.
Windows Explorer lists all partitions of the PC. Check the system reserved partition on the affected PC to find out more about it. You may notice that it has a size of 100 Megabytes and that most of the space is not free.
The system partition had 15 Megabytes of free space on the Windows 10 PC that threw the error message.
Windows updates may fail if the system partition does not have enough free disk space. Windows administrators can correct the issue by freeing up disk space.
I managed to free up 15 Megabytes of space by deleting all language files but the language file that I needed and running check disk to truncate the NTFS log. This resolved the Windows couldn't be installed. We couldn't update the system reserved partition.
Microsoft recommends deleting data on the system partition to free up disk space to free up 15 Megabyte of space (when upgrading from Windows 8.1 or earlier), or 13 Megabytes of space when upgrading Windows 10 to a new feature update.
You cannot just access the data on the system partition and delete the fonts folder as the partition is not mounted to a drive letter and because of a lack of permissions.
Note: You may need to display hidden operating system files. See Show hidden files in Windows 8 and 8.1 for instructions. These work for all versions of Windows.
Use Step 1 from above to determine the partition type. The following instructions are for Windows 7 or Windows 8.1 machines with GPT partitions:
Note: You need to have an external drive with at least 250 Megabyte of space.
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