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The question came up while I was thinking to move some data (e.g.) pictures, old but important other data solely to the NAS (private use, no business level).
My NAS is a simple 2 disk RAID system. I know that a RAID is not a backup system as it only provides hardware redundancy so I planned to create incremental (versioned) backups of the data directories. The backup destination would be the same NAS so that the only instance of the data (no copies on notebook or desktop machines) and the versioned backups would stay entirely on those two disks.
Now I wondered how reliable this procedure is. I've read that a power blackout has typically no effect on the data on a disk, as the heads retract to a safe position and do not "hit" or "scratch" the disks, but it can of course corrupt data currently written and even worse affect the electronics of a drive rendering it unusable.
Now I am pretty unsure whether I should invest in a small UPS that plays well with the NAS to ensure proper shutdown or resort to multiple backup locations (or both?)
Does having only one instance of data + backup on same machine still regarded unsafe?
(I am ignoring theft here. Also surges, as all connected cables LAN, Power etc. are protected. Even more: I do not want to use the cloud for any data (encrypted or not))
Edit: Sorry, it is a RAID 1 with two disks total.
RAID1? To be properly backed up, there should be 2 backups (on & offsite). But that depends on how critical your data is... you could add another external hd that you backup manually every so often but you remove from nas/desktop and store elsewhere. At the very least disconnect. – Logman – 2014-08-03T13:43:53.030
You don't mention the which flavor of raid that you have... RAID0 (striping) provides speed with no additional data protection while RAID1 (mirroring) and RAID 5 provide drive redundancy. At home, I have 3TB of RAID1 plus crashplan as my offsite copy. – Brian Adkins – 2014-08-03T13:46:44.360
Bit rot (or the random flipping of bits due to cosmic radiation) is growing problem for our 4tb disks. Without some form of checksumming (such as ZFS provides), bitrot can invisibly melt all of your files. – RoboKaren – 2014-08-04T04:30:24.913
Michael already provided an excellent in-depth answer but I wanted to note that what you describe in your question is not a backup but simply a versioning system of your working copy of the data. It's a reliable solution against human error and file corruption but doesn't match the requirements of your scenario. – Lilienthal – 2014-08-04T08:29:03.587
3Lightning, flooding, fire, theft. Unless you have offsite backup any of these can wipe out both sets. – JamesRyan – 2014-08-04T10:03:21.573