After doing a clean install of Windows 8 my Seagate 3 TB HDD won't reactivate in Disk Management. The two volumes are there but I can't use them for some reason. The drive was previously used with a GPT partition table, I can see the two spanned volumes but can't reactivate either.

I backed up all my files from Windows 7 onto that drive and desperately need them back. What can and should I do to get the drive back up and running?

When I go to Disk → Properties → Volumes, it claims the drive has a MBR partition style, so converting to GPT somehow without data loss should work. gDisk claims to be able to do that but when I point it to the drive, it claims that it has a GPT partition and a protected MBR partition.

Any suggestions on what to do?