The GUID Partition Table (GPT) is unique in that it resides both at the
beginning and the end of a disk. Its removal is not as simple as zeroing
out the first 512 (or 446) bytes.
For demonstration, I have a Seagate 750GB drive with a device name `sdb`
Figure out where to start zeroing
[root@livecd centos]# fdisk -s /dev/sdb
732574584
This is the total number of blocks; we'll zero out the last 100,000
blocks:
dd if=/dev/zero of=/dev/sdb seek=732500000 bs=1k
Now zero out the first 1000 (overkill, really)
dd if=/dev/zero of=/dev/sdb count=1000 bs=1k
Ta da!
Sources
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* [GPT Removal](http://www.digital52.com/help/gptremoval.html)