That's a weird problem but other people seem to have had issues like that:http://www.macosxhints.com/article.p...60718140450785http://www.macosxhints.com/article.p...70628092633507
I don't recommend following the steps in the first one as that involves installing a third party EFI shell but I just posted it to show people have had similar problems.
I would try to use the USB prober utility from the developer kit and try to get the drive to mount that way. I uploaded it for you to save you getting the developer kit - it should run without the full kit:http://www.sharebigfile.com/file/205...rober-zip.html
To download, you have to put in the number, hit submit and wait 30 seconds.