When using Robocopy to backup data to a NAS, I always had the problem that most files were considered “old” on the NAS device (even if they weren’t) – that kind of defeats the purpose of the /MIR switch.
Today I finally decided to search for a solution – and it was remarkably easy. Most NAS devices use Samba or something similar to provide “NTFS shares” – but most of them only implement FAT-style file times with a 2-second-granularity.
You can force robocopy to use FAT file time as well using the /FFT switch. Now my backup script works again as expected.
See also here.