Installiere "gdisk":
apt-get install gdisk

Entferne die Mounts vom Datenträger und starte gdisk:
gdisk /dev/sdX

Der Prompt sieht dann so aus:
 _____________________________________________________
| Found valid MBR and GPT. Which do you want to use?  |
|  1 - MBR                                            |
|  2 - GPT                                            |
|  3 - Create blank GPT                               |
|_____________________________________________________|

Wähle hier GPT, also Nummer [2].

Command (? for help): [x] 
 _______________________________________________________________________________
| Expert command (? for help): [z]                                              |
| About to wipe out GPT on /dev/sdx. Proceed? (Y/N): [y]                        |
| GPT data structures destroyed! You may now partition the disk using fdisk or  |
| other utilities.                                                              |
| Blank out MBR? (Y/N): [y]                                                     |
|_______________________________________________________________________________|