Thanks for the help guys. Double checked that both new drives were set to Master (dual IDE channels makes things easy), refitted the original Primary (rear) drive, and away it went - formatted the new sec drive in about 15 min. Shutdown and swapped original prim drive with the other 250 and all went well. Once all that was done, set Raid to 1, and about 40 minutes later, had a mirrored 247GB. I'm happy!! So my config is now:
System: 3.4.805
Hardware: 2.0.0
Serial: 48xxx
BIOS: 2.0.252
RAM: 128MB
Built: Apr 5 2001
Drives: 2 x Western Digital WD2500JB 250GB
Config: RAID-1 (Mirrored)
There is so much info on this site that is helpful, it is also easy to get info crossed up, which is what I did. Once again, thanks. You guys are great.