Thanks to all for the help.
The drive does checkout OK with Check4GB.
I moved most things off to the external HDs, then repartitioned (again into five) and reformatted the internal HD, reinstalled OS 3.1. All seemed to be well, but on reboot the supposed boot partition still showed up as a non-DOS device? All the other partitions are as they should be!? If I look at it with HDtoolbox, it's shown as bootable and seems to be fine.
I'm baffled (and getting crazyish).
Thanks again.