Check the account that the used to fire off the scheduled backup, it may just be the scheduler service. Chance are, that account is the one that is active and being used to connect to the NAS. If so, creating that account with the same username and password on the NAS should do the trick.
Personally, I would suggest Get a different backup program -- I don't know much about Windows Server Backup on SBS, but I know that dedicated backup programs are specifically designed for the kind of workflow you're describing.
If you want to try to save costs I'd suggest you shelve your current backup drives and buy new ones, then configure a proper backup rotation.
A "proper backup rotation" would look something like this:
- Every Friday take a full backup of the server. Send that disk off-site on Monday morning.
- Monday through Thursday take an incremental backup - ideally to separate disks like you're doing now - and send those off-site.
You would need at least 6 disks (Monday through Thursday incremental backups, and two Friday "Full" disks) for this process, and would scratch each disk as you reuse it.
The idea is to always have one "Full" backup set off-site that will allow you to restore to your current state (either last Friday's full backup, or the previous week's full set of disks).
Basically each disk is treated as a (very large) virtual tape in this situation. Depending on how much data you have to back up you may be able to store several weeks of backups on the disks by configuring the backup software appropriately.
As Grant pointed out in his comment, YOU ALSO NEED TO PERFORM RESTORE TESTING when you set up your new backup rotation.
Based on your question I can't tell if you do restore tests regularly, but it sounds like you don't (otherwise you would know for certain if you really need all of your disks for a restore). Backups that have not been restore tested effectively don't exist, and testing them in an actual emergency where you need to recover the system is a Bad Idea because if they don't work you're in a really bad situation.
The usual recommendation is to perform a restore test quarterly, or any time you change the backup set or system configuration.
Best Answer
After googling a lot - I have found the answer. It seems that the performance hit is the whole time the server is running and not just during backups. This is because all writes to disk are made twice - once to the disk and once to a "copy of changes list" (to simplify). If the server is mainly readonly then that isn't a problem. if it is read/write then disk access is 2x slow.
https://social.microsoft.com/Forums/en-US/607809bc-8495-4d7e-93a3-0f42f5a4dff2/optimize-backup-performance-setting-under-server-manager?forum=whsvailbeta