the output for sgdisk --zap-all /dev/md/raid1 was

Warning! Disk size is smaller than the main header indicates! Loading
secondary header from the last sector of the disk! You should use 'v' to
verify disk integrity, and perhaps options on the experts' menu to repair
the disk.
Warning! One or more CRCs don't match. You should repair the disk!
Main header: OK
Backup header: OK
Main partition table: OK
Backup partition table: ERROR

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Caution: Found protective or hybrid MBR and corrupt GPT. Using GPT, but disk
verification and recovery are STRONGLY recommended.
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GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.