Hi, Gonzo!
I might be incorrect, but I would try the Western Digital with the Single setting. Unlike others, WD specifies the "Master (dual)" setting when the drive is installed along with another drive on the same bus, which isn't the case here. The Single setting is akin to the single Master setting on other drive brands.
If you're having problems with the drive, you might consider foregoing the external case setup for now and instead install it in the Mac and see if you can mount it/repair it there first.
Have you or can you run Repair Disk on the drive with Disk Utility (DU)? If so, what happens/what error message do you get? Have you tried to mount the drive using the DU "Mount" option? Have you tried resetting PRAM? Can you mount the drive booted from OS 9 or the OS 9 install disk, using Disk First Aid and /or Drive Setup (don't initialize!)?
DiskWarrior is arguably the best drive directory repair utility to be had, and might be able to rebuild the directory and restore the drive, if the above doesn't help. If DW won't, however,
Data Rescue II may be able to recover the data if the drive can't be repaired. You might download the free trial version and see if it "sees" the files on that drive. If it does, it most probably can salvage them.
Gary
1GH DP G4 Quicksilver 2002, 400MH B&W rev.2 G3, Mac SE30 Mac OS X (10.4.2) 5G iPod, Epson 2200 & R300 & LW Select 360 Printers, Epson 3200 Scanner