The VSS Diff failed

The VSS Diff failed(An error occurred during fast difference check. Slower method will be used instead) issue occurred

This issue is usually caused when the shadow storage size is too small.

Use commandline and perform the following command to resize the shadowstorage:

vssadmin resize shadowstorage /for=X: /on=X: /maxsize=320MB

X -the disk letter (best practice is to perform the operation for all the accessible disks).

vssadmin resize shadowstorage /for=X: /on=X: /maxsize=30%

After the changes, perform the backup task again. If that won't help, then try to delete all the shadow copies using two additional commands in the commandline:

diskshadow

And after that:

delete shadows all

Perform the backup once again, if it still fails, then contact our technical support by submitting a request: HERE