PrimoCache's write delay was set to 5 minutes. I had just checked my mail (Outlook) and quickly made an image backup, as I do every few days. After restoring the backup to another volume, Outlook.pst was corrupted beyond repair.
Cause: Macrium Reflect reads physical disk sectors, seemingly bypassing PrimoCache's disk driver. Part of the modified Outlook mail file was still in PrimoCache's write buffer and hadn't made it to the disk yet. Checking the original partition, the .PST was fine. Also, testing the backup showed no error because all it does is check the archive for readability. It does not have a compare option.
Solution: turn off the write delay before imaging. Simply flushing the buffer is not enough, since any part of the partition may be modified at any time on a running system.
Interesting that it's possible to use a system for years without a single problem and suddenly discover that I've been making potentially worthless backups all the time. Be warned.
