Assume a small office situation, 5-10 workstations (PCs), and a Windows server hosting a network share. The Windows server is in a 3+1 RAID 5. I am debating with myself over a backup solution and I find myself thinking about it way more than I should. I can:

* Stick an additional large disk in the main server. Make backups from the PCs/share to it.

This is surprisingly common and a lot of places do it but it kind of scares me since the chassis (lightning, MOBO fault) can take out all the disks.

* Separate backup server with single large disk (no RAID)

This seems like a good idea. Buy a cheap low end PC and stick a 2TB drive in there. The chance the live data goes out and the backup server\drive dies at the same time is low. It's still not a RAID though

* Cheap mini NAS in RAID.

Not sure if this is better than option 2. Yes RAID makes things more redundant however RAID also increases the AFR (more disks to go bad).

Performance is a non issue since this is a small office backup that will run off hours. For increased reliability I can couple this with Amazon cloud storage.