Eu encontrei a solução aqui . A parte básica é:
OK, then, it looks like you've got a new MBR with residual GPT data, which is confusing GParted. You'll have to wipe the GPT data. Here's how:
Boot your PartedMagic or System Rescue CD emergency disc.
For safety, back up your MBR by typing "dd if=/dev/sda of=sda-backup.mbr bs=512 count=1" and then copying the resulting file (sda-backup.mbr) to a USB flash drive, network share, or some other medium. Be sure to get the if= and of= parameters right, though!
Launch gdisk by typing "gdisk /dev/sda". When asked whether to use the MBR or GPT data, either will work.
Type "x" to enter the experts' menu. The command prompt should change.
Type "z" to "zap" (destroy) the GPT data. Tell it to proceed, but answer "N" to the question about blanking the MBR.
That's it. Windows should boot as it did before and you should now be able to continue your installation of Ubuntu; its installer should now see the same partitions as Windows.