How to complete an unfinished backup

I created a new backup today from and to hard disks attached to my computer. It took two hours and did not complete for some reason. (I’ve tried reading the log but don’t understand much of it.)

In my dashboard it says Source 239 GB, Backup 222 GB / 1 version.

This looks quite clearly that some files were not backed up. What should I do? Re-run the backup, or remove the initial, incomplete, backup, to try to make a complete one?

I tried running verify, which passed.

The backup is being compressed. This why the source space is higher than the backup one.