The beauty of most NAS solutions are that they BOX is independent to the choice of the drives you put in that box. So, you make the choice of the size of NAS by two factors.. How many drives you want to put into the unit, and how big those drives are.
A single drive is just the size of the drive minus any overhead for the NAS O/S.
But what if you want larger and don't want to buy a really big capacity drive. Then you get into RAID. (Redundant Array of Inexpensive Disks) What this can do for you is either expand the capacity of the drives, give you drive failure redundancy or both. Depending on how many drives you have, RAID can give you many options.
With 2 drives, you have either RAID 0 or 1.
Raid 0 is striping where part of files are written to one drive and another part to the other. You get the capacity of both drives added together and a bump in drive performance, but with a risk. if either of the drives dies you loose everything as there is NO fault tolerance.
Raid 1 is drive mirroring. Where whatever is written to the first drive is automatically done on the second. You only have the capacity of a single drive, but if one of the drives fails (hardware failure) you don't loose any data. In such a case you can replace the failed drive and the RAID will re-copy all the data onto the new drive to restore the fault protection. You can use this method to also increase the size of your drives buy replacing one of you drives with a larger new drive and having it rebuild then replace the other.
If you have 3 or more drives then you can go RAID 5, that takes the idea of raid 0, but adds in fault tolerance buy writing a parity block. In raid 5 your capacity is (n-1) where n is the number of disks you have.. so for example I have a 4 disk raid 5 with 3tb drives. So I have (4-1)x3tb=9tb of drive space. If one of the 4 drives fails (hardware) the parity block can be used to figure out missing data from the failed disk.
The only thing that you must remember is that RAID fault tolerance only covers you for hardware faults. If you have a RAID 1 and you delete a file off your NAS, it would delete the files off both drives. If your computer barfs while writing to the NAS and the files is written corrupted, then it would be written corrupted to both drives. RAID is not the same as duplicate backups. So dont think of a RAID solution as well I have a backup of that as I have two drives in my NAS.
I have 3 NAS units. One NAS is my main drive where I store all my Media, Documents, Photo's, CD Rips... Then the other two NAS units backup the really important stuff that is on the first NAS. As the data is really stored in 3 different places, I have backups. Also, each of the NAS units have fault tolerance so the data saves on them has hardware failure protection as well.
I also have a SATA dock hooked up to the first nas where I will periodically backup all my important data for archive and take it off site (to my parents house) so that just in case of a fire/fload/breakin my important data is protected.
I looked at it as the cost of trying to re-produce my work would be multiple factors more expensive than the cost of hardware that went into this solution. But it's been bought over a 3-4 year period.
