Just an update. Good news!
I've been running the RAID1 setup for several days now with no problems. Today I added the other two drives into the NAS. What I did was:
- turn the unit off
- add one drive
- use the webUI to format the drive (as a standard drive)
- use the storage utility (on the CD) to map the drive
- repeat for next drive.
Both additional drives were recognised fine, formatted fine, mapped fine.
JavaLawyer, once again, thanks!