Since a couple of weeks I noticed that my Time Machine did not backup. I observed that this began more or less the same time when I had rebuilt my Spotlight index, because I had some problems with Spotlight that keeps on indexing my HD and as a consequence slows down my Mac.
I Googled around to find a solution and although I found some possible solutions I still have the problem.
I have also a notebook backing up to the same Synology and this backup is fast as ever.
After many hours the backup is completed and now it seems stuck for hours in verifying the backup.
Finally the backup was completed but the verification took a long time as well and was stuck on 69%.
I am suspecting a hardware problem, possibly the harddisk.
Diskutil did not report anything strange.
My HD is a Seagate HD thas qualifies for the replacement program, I missed the 3 year deadline, so now I am stuck with this disk.
You can try to run some standard diagnostics tools to eliminate some basic causes.
Time machine diagnostics
Next step is to run the Apple Hardware Test (AHT).
Reboot and press D before the grey screen.
I noticed from the activity monitor that there were several postfix background processes running. These were initiated because I had OSX Server running on this machine as well. Although the email service was not activitated in the configuration, it seemed that these processes were running altogether.
I decided to deinstall OSX server and see what would happen.
After deinstalling OSX server it seems that my Time Machine backup is running as normal.
My analysis is that these postfix processes caused some constant file io which interfered somehow with Spotlight/Time Machine.