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The practical case is more complicated...
GPT uses the beginning and the end of the disk to store metadata. It can vary depending on the original OpenXT install, but gdisk usually finds enough room for itself In general, there's enough free space at the beginning of the disk (i.e. before the first partition) for gdisk to store the GPT. However, usually, the OpenXT LVM and partition are configured to span to the very end of the disk, leaving no room for the secondary GPT table. We need tricks to make room.
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