I'm a little bit rusty at WB2.05 installs, but are you sure you have a filesystem that the 2.05 ROMs can understand installed on the drive? If you install it on an A1200 you'll get the 3.0 version of FFs installed, and it won't boot on the 600... Make absolutely sure you've got hte 2.05 version of FFS in L: on your boot floppy before you install it with HDToolbox...
Failing that (and I don't want to insult your intelligence here), but you did check the automount option in HDToolbox? It may or may not be stored differently between the WB2 and WB3 versions of HDToolbox, so checking it on the 1200 might not necessarily mean it'll automount under WB2...