It depends on how the RAID is set up.
Some, including many hardware cards will do this:
- Stripe (RAID 0) over both drives, just concattenating both drives: 3 TB usable.
- Stripe (RAID 0) over both drives: Using 1TB on both drives: 2 TB usable.
- Mirror (RAID 1) over both drives: 1 TB usable.
Some (such as mdadm RAID using in Linux and on many SOHO NAS devices) allow you to do this:
1 TB drive: 1TB data in mirror
2 TB drive: 1TB data in mirror + 1 TB data as just a simple volume
I know your NAS does both striping and mirroring, but not the precise details.
1) Since the default formatting is in ext3 it is probably using some form of GNU/Linux.
2) Page 32 of the manual seems to allow using part of a drive as part of a mirror.
So, if I then went out and bought a 2 TB hard drive to replace the 1 TB hard drive will I have to wipe and repartition the hard drives? Or will I be able to just swap the 1 TB for the 2 TB and magically have 2 TB of space?
If you replace the old drive: Yes, you will have to repartition and you will have 2 TB.
If you just add the second drive in addition to the existing 1TB drive, then you can:
- Add second volume so you have a 1TB and a 2TB volume on the NAS.
- Or backup all data, reformat the NAS as a 1TB mirror and a 1TB JBOD.
(Then copy all data back. The data on the mirror should survive even if a single drive fails).
- Or backup the data. Add the second drive and configure it as a 3TB stripe. This will be faster, but you will loose all data if any drive fails. And the speed gain is probably negligible because the network will be the slowest link.