worrying warnings issued by rsync

Today when I rsync-ed my Mac’s disk to my Synology NAS-s, I found a couple of lines like this one:

WARNING: abc.csv failed verification -- update discarded (will try again).

file has vanished: "def.csv"
IO error encountered -- skipping file deletion

Of course this worries me. I stopped the script, that starts the rsync-s. I ran it again. The warnings did not show up again. I searched the web for this warning message, and I decided to check the disks’ health of the (mirroring) disks on the NAS and the Mac in question.

On the Mac: Applications > Utilities > Disk Utility – I selected the entire disk, selected First Aid and then Verify Disk. Everything appears fine.

On the Synology NAS: Storage Manager > HDD/SDD > Health Info – I scheduled a weekly “extended S.M.A.R.T. Test”.

All my Synology NAS-s create a “Monthly Health Report” anyway, being delivered via e-mail.