External 3 TB disk.
Stopped working after a power bump.
GPT reports as follows:
bash-3.2# gpt show /dev/disk6
start size index contents
0 1 PMBR
1 1 Pri GPT header
2 32 Pri GPT table
34 6
40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
409640 2930266584 2 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
2930676224 262144
2930938368 2929332616 3 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
5860270984 262151
5860533135 32 Sec GPT table
5860533167 1 Sec GPT header
This looks normal.
bash-3.2# diskutil repairDisk /dev/disk6
Nonexistent, unknown, or damaged partition map scheme
If you are sure this disk contains a (damaged) APM, MBR, or GPT partition map,
you can hereby try to repair it enough to be recognized as a map; another
"diskutil repairDisk /dev/disk6" might then be necessary for further repairs
Proceed? (y/N) y
Partition map repair complete; you might now want to repeat the
verifyDisk or repairDisk verbs to perform further checks and repairs
Repeating this makes no change.
DiskUtil reports as follows:
bash-3.2# diskutil list /dev/disk6 /dev/disk6 #:
TYPE NAME SIZE IDENTIFIER 0: *3.0 TB disk6
I figured that it might be a bad PMBR. From all accounts, this is just an MBR with a non-bootable partition in it. I created an HFS disk on a USB stick, and copied the MBR (dd if=/dev/disk7 count=1 of=/dev/disk6) to the ailing disk.
I used R-Studio to scan the disk. R-Studio finds files and directories and such.
Ran TestDisk (separate command line disk utility) It's numbers agreed with GPT's.
I then used dd to make a backup of the first 34 blocks of the disk, and used gpt to destroy and recreate the partition table.
604 gpt destroy /dev/disk6
605 gpt create /dev/disk6
606 gpt show /dev/disk6
...
610 gpt add -b 40 -s 409600 -i 1 -t efi /dev/disk6
611 gpt add -b 409640 -s 2930266584 -i 2 -t hfs /dev/disk6
612 gpt add -b 2930938368 -s 2929332616 -i 3 -t hfs /dev/disk6
613 gpt show /dev/disk6
No joy. I'm missing something here.
Best Answer
The problem turned out to be the PMBR. My first attempt was to copy sector 0 off a GPT formatted memory stick.
Examined that with fdisk and it had partition of type EE, but start and end cylinders were the same.
I edited partition 1 and accepted defaults.
fdisk on mac will report "can't get exclusive access to write sector back, reboot required" It lies. You don't have to reboot.
After doing this diskutil was able to see both partitions.
I was unable to repair the first partition, used for time machine, and ended up reformatting it. (Even after it was a recognizable partition, diskutil would start on it, and give up in a few seconds.) (I didn't try very hard. The really important stuff is stored on drop box, on a mirrored disk on another machine.)
@Klanomath deserves credit for GPT disk. I had found this, but missed the fact that it provided Mac binaries. I'm not used to binaries on Source forge. GPT disk appears to be a useful tool for dealing with this sort of situation. It would have been a better way to go.