OK - call me stupid.  I just noticed an extra drive under My Computer (Drive E:).  It is loaded with the previous owner's data and stuff (some pretty racey stuff, I'll bet).  Therefore - I conclude it was partitioned.

Now how do I get rid of this so I have only two HD's?

Firstly, to prevent futher problems, make sure the two drives are set to cable select, not master/slave; the latter can cause problems with disk detection and formatting (as you originally discovered) on GX1s with original cables.  Make sure both disks are set to AUTO in the bios.  To "recover" the extra space on the second disk, you'll need to delete the old partition you no longer need (once everything is off it) and either reformat the whole drive as one or, if you need to retain data on the first partition, use product such as Partition Magic to resize the existing partition to cover the whole disk. Make sure you back up everything that's vital first though if going this route.