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My thoughts are starting with a 4 bay enclosure running 4TB WD RED drives. My desktop (editing machine) has faster 7200rpm HDDs where I would import currently 'active' work to which would be edited, post processed, and backups sent to external HD's. Once the files are finished being edited & selects sent to clients they would be moved to the NAS for longer term storage/access freeing up the desktop drives. Not certain which RAID configuration I'd use as of yet.
If anyone see's something flawed with that plan or has a better alternative I'm all ears, though I am on a budget.
I don't see a flaw(as such) but consider the implications of RAID!
RAID 0 you don't want .. simple as that.
RAID 1 and 5 are similar, but different enough to warrant their existence.
Problem is the wasted disk space if the need ever arises.
I have 4x 4T WD Reds myself in the QNAP, and each individual Disk allows me a few years of storage headroom.
Currently have 1.6Tb of image storage needs, and I estimate that 4Tb will last me about another 5-10 at least.
So in 5-10 Years time, I'm allowing the need for larger (say 8 or maybe 16Tb) drives by then. in the years I have what I have, they'd have 'paid' for themselves in the work they'd done for me.
Problem with raid:(eg. RAID5) you need 3 disks to cater for the space allowance of 1 disk. Great that they're safe with parity and pretty much disasterproof relaiability .. but the wastage of requiring 3 disk for the privilege having one disk at your disposal isn't what I'd called efficient.
In your situation above 3 of your 4 4TB drives that total 12Tb will only give you 4Tb of usable space! The last 4Tb drive will hold the NAS box's OS and you can store other data there too. so you have just under 8Tb of storage space available.
So what I've done, is a psuedo RAID 5 but manually done.
2x 4Tb drives on the QNAP(ie NAS box) and the remaining external USB drive still hooked up the PC.
(Originally I had 2x USB drives on the PC as my 'backups')
The USB disk at the PC is USB3 and so data transfers are fast(100+Mb/s) so accessing images is painless .. no waiting for loading and so on.
The 2x 4T drives on the NAS is simply for backup purposes. I rarely access them.
In fact the way I have it setup, is that one of those drives on the NAS is mapped in Windows, so that I can access it easily, but the other is not.
Reason is for security, if the need arises. eg. if I accidentally attract some virus/trojan that scans all drives and deletes/destroys data(my sister had that happen) at least the one drive is unaffected .. it's on an alien OS relative to the Windows malware.
*Note that this above scenario is unlikely, in that I've never had a virus of any kind(I'm one of those cautious types that avoid rather than curious) .. but it's a 'plan I have'
Anyhow.
So my backup routine consists of:
Take images, download to PC.
On the PC I use a 1Tb SSD for the days images. This drive only stores images for the year. Every year they get deleted and I start again.
The 2TB USB drive on the PC is my archive. That drive is the one that is backed up on the 2 4Tb drives on the NAS. Both manually done. the networked drive is backed up regularly(as it's easy to do because that drive is mapped)
The non mapped 4Tb drive on the is the failsafe(as long as it doesn't physically die!!)
I only back this one up at half yearly or yearly intervals.
Main reason for that failsafe backup drive is that a while back I was in auto dumb mode(as we sometimes tend to get into) and was regularly backing up HDD to HDD of my images.
Little did I know, that at some point there was some data corruption of some of those NEF files. So I had no idea that I was backing up corrupted data. I lost about 20-30 NEF files, but I don't think they were a major loss.
So this unmapped drive is my 'snapshot' drive. before I backup to it from the mapped NAS drive, I thoroughly check close to every NEF file via whatever software allows me to do so quickly.
I used to use ViewNX in raw mode, but it's slow over the network connection, so I tried Microsoft's Photo Gallery(works well enough) and then I tried XNViewMP fast over the network connection.
This involves a very brief manual inspection of each thumbnail of the NEF images.
I don't know of any program that can verify the integrity of each NEF file automatically.
When I do a backup/sync of the drives, I use FreeFileSync to inspect the differences between each drive. It just displays if there are, and what differences there are. But I use FastCopy to do the actual backup/syncing duties, using the 'verify' option.
Reason I use FreeFileSync is that I'd like to see visually what, if any, differences there are ... rather than just do a backup for no reasoning.
RAIDs take a while to build up. and if you have to replace a drive, you have to rebuild the raid again. If you need more storage space, you need to update all drives, as the storage space is limited to the size of the smallest HDD.
I have close to 16Tb of flexible storage on my NAS box, if I needed backups of more than 4Tb of data, I can easily acquire just a single >4Tb drive and use that .. singularly .. and update the other drives singularly as needed.
(you did make the point that you are on a budget!).
While three individual HDDs acting as a pseudo RAID 5 is more work, and not quite the same in terms of being failsafe, I think the prospect of those 3 drives all physically failing at the same time is too remote to concern myself with.
I used to have 2 x external USB drives connected to the PC, and when the one failed about a year ago, I looked for the NAS solution rather than to replace it .. so I've had HDD failures(usually Seagates too!).
Not a good feeling only having to rely on the one USB drive, that Murphy's Law dictates is likely to go wrong just when you least need it too :p