If anyone is interested, using the command line mode in Clonezilla, I removed the GPT components using gdisk:

# sudo gdisk /dev/sda
GPT fdisk (gdisk) version 0.7.2

Partition table scan:
MBR: MBR only
BSD: not present
APM: not present
GPT: corrupt

Found valid MBR and GPT. Which do you want to use?
1 – MBR
2 – GPT
3 – Create blank GPT

Your answer: 1
Command (? for help): x

Expert command (? for help): z
About to wipe out GPT on /dev/sda. Proceed? (Y/N): y
GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.
Blank out MBR? (Y/N): n <— do NOT choose Y!
MBR is unchanged. You may need to delete an EFI GPT (0xEE) partition
with fdisk or another tool.

answer copied from this thread.