My 1TB RAID 1 was getting too small, so I put new drives. I used QNAP’s admin interface to extend and rebuild the RAID. It now has 3.6TB.
However, UrBackup still “sees” the old RAID size.
Rebooting the NAS doesn’t help.
Is there something I should do?
The space managed by the RAID may be 3.6TB, but what of the volume managed by the OS? What did you do to resize the filesystem?
After more closer look, everything is fine.
It was my mistake 
After looking at the Statistics tab, I saw 0-900 GB and assumed UB didn’t detect the new size.
Now, however, the numbers on the chart increase as the size of backups increases.