Backup is not fit for purpose
The backup to USB drive feature is seriously buggy. You cannot interrupt a backup and then have it pick up from where it left off. It just starts all over again.
Much worse still, if you have a scheduled backup, but the drive is not connected when it starts then next time you do a backup it also starts from scratch. It also deletes all the files of any previously successful backups! Not good.
Finally, unless your external drive is formatted ext3, it will take literally days to backup just a couple of hundred gigs. Couple that with not being able to restart failed or cancelled backups from where they left off and you are in big trouble unless you can leave it on 24x7 for days at a time with the USB drive connected. Kind of negates the usefulness of having an offsite USB drive backup.
One other gripe. It starts thumb nailing any pictures you put on the drive. This took two weeks (yes you heard me right) to complete with 2500 pictures. The thumb nailer process burns all the CPU, so whilst it is going on the transfers rates are right down. The only way round this is to manually ssh into the NAS and stop the thumb nailer service manually.
Shame, because it's basically a nice box.