The error VSS_E_FLUSH_WRITES_TIMEOUT may occur in various circumstances. This error usually means the preparation of a VSS shadow took too long. The timeout is fixed in Windows and can’t be changed. Usual suspects here are heavy fragmentation on C: or having too many shadows in the system.
Please check using
vssadmin list shadows
there should be none. If there are shadows, delete them with
vssadmin delete shadows /all
Then run a defrag on c: (and the affected drive/s) and ensure there is at least 50GB free space.
Additional ways to prevent these errors are the following. On PCs, turn off system protection for all drives:
on Win 10 the screen looks like this:
If it’s a Windows Server, use vssuirun.exe and remove all shadows and delete schedules on all drives.
Then the final question is what kind of backup are you running? Note that the type shows up in the Progress tab and can’t be changed once the task is created. If you need to back up just documents, please use ‘general file backup’ task types. These are easier on VSS. If you need to back up Hyper-V or VMware, use the corresponding task type. ‘Universal’ type of task types should only be used when necessary, as they are heavy on VSS because they involve all installed VSS writers.
Finally, this type of error is rare on SSD drives. If you use mechanical drives, it could be the drive is too slow relative to the load it has to handle. If all of the above doesn’t resolve the error, you will need to change the backup time to a more quiet time or separate VMs or other heavy I/O users into different hard drive partitions or ideally store them on different physical drives.