You need to write zeros to LBA 0 and then reconstruct the GPT Protective MBR and Partition Table.
To fill LBA 0 with zeros ...
Edit -> Edit Mode
Mode -> Hexadecimal/Text
Tools -> Fill Sectors
Pattern - Hex - 00
Start Sector - 0
End Sector - 0
Number of Sectors - 1
To reconstruct the partition table, write the following data to LBA 0:
- Code:
Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
000001C0 02 00 EE FE FF FF 01 00 00 00 FF F6 37 3A 00 00
000001D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000001E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000001F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA
Select Drive -> Apply Changes.
I have followed the instructions in the following document:
An Examination of The Microsoft Windows 7 and 8 GPT 'Protective' MBR and EFI Partitions:
http://thestarman.pcministry.com/asm/mbr/GPT.htmPlease clone your drive before attempting the above. The procedure applies to a Windows data drive, but I'm not sure if Apple is any different.