Had similar problem with new Astone ISO Gear external drive.
Recognised OK with Vista Home Premium, and had files loaded onto it, but when attached to Windows XP system, only appeared in 'Disk Management' as 'GPTprotected drive' and shown as empty.
When plugged into Vista sytem on another computer, all files still shown.
After much searching, came across 'Paul Gu' blog, with reference
Using the information provided, I first copied my files onto Vista hard drive, connected the external drive to the XP system, then using 'diskpart' from 'Run", carried out formatting as detailed in blog.
When formatting etc. completed, I had a 'System Information' folder on my external drive.
I re-connected the external drive to the Vista system, deleted the 'System Information' folder, copied my saved files back again, and all is well now.
I can use the external hard drive on both Vista and XP, it is stated this also works for Mac users.