Inexpensive NAS for over 4TB backups - recommendations?

The upgrade on our LA-based Pro system is complete and I've begun backing up to our 104 backup NAS.

I set up two backup jobs:

Job 1)
source - volume C (ReadyNAS Pro)
destination - remote NSF volume /data/Backup (on the ReadyNAS 104)

Job 2)
source - volume C (ReadyNAS Pro)
destination - remote RSYNC volume Backup (on the ReadyNAS 104)

I tested and verified both jobs were set up and working properly

I ran Job 1 as a full backup. It took about three days and then failed with an unknown error. There is no information in Job 1 log on the cause of the error. I checked the backup volume on the 104 and 2.94GB of 3.06GB has been successfully copied. I did a spot verification of the files and they all seem good. All critical shares and directories seem to have been backed up fully.

I don't know why NSF failed but the 104 is still responsive and at less than 50% capacity. I also upgraded to 6.1.5 before starting this process.

Since 90% of the backup was complete, I started the RSYNC job to initiate regular incremental backups.

Questions -
When setting up a backup job, it appears that a full backup must be performed at least once at the start. According to my logs, RSYNC is doing a full back-up but is first sending an incremental file list of every file in volume C.

Is this normal?

Is RSYNC actually doing a full back-up all over again? Or is it simply building a list to compare against what's already on the backup NAS? I'm hoping it copies only the files that didn't get copied with the first NSF backup.

Thanks!