What do you mean you BIOS hangs? If you are getting the messages below then your BIOS had passed control to Chameleon.
Thanks again for your input.
When I say the bios hangs I mean that if I format the disk as GPT and plug it it and reboot I get as far as pressing F11 to get the boot menu, the bios tries to read the disk - the red led comes on - and stays on - and nothing happens. It hangs. I have to unplug the disk to get the boot to continue.
The strange things is that if I boot into Pmagic without the disk plugged in, get PMagic running and then plug the disk in it is recognised as the GPT disk, with all the data intact.
The problem is that I can't boot with the disk (formatted as GPT) plugged in.
If I format the disk as FAT32 the red led flashes briefly and I get to the boot menu.
I only get the boot0 messages if I overwrite the GPT MBR record with a FAT32 one. Seems like a bios bug or USB incompatability. I'd just like to find out what the bug is - I suspect the USb as there are plenty of MSI Winds out there running OSX. I'm just trying to find a way round it.