I have an external USB3 hard disk which used be my Time Machine backup drive.
It is not mounting in Finder, but is showing up if I
diskutil list. It sounds like it wants to mount - in fact, it sounds like a drive where there is data being read and written.
If I open Disk Utility with this drive connected, I get the spinner icon and Disk Utility stays a grey window and won't list anything (ever). I have to force quit or reboot to quit it at that point.
fsck_hfs it from terminal under the OS:
Tims-MBP 08:48:50: [~]: fsck_hfs -r /dev/disk3s1
** /dev/rdisk3s1 (NO WRITE)
Can't open /dev/rdisk3s1: Permission denied
but if I reboot into single user mode then it lets me.. the process ticks by and tells me about things like
Previous ID in a hard link chain is incorrect,
Orphaned file hard link. and
Invalid volume free block count. which (after a few hours) it seemed to have a grip on. (probably bits of the time machine backup are corrupt: that's acceptable and expected).
I also thought to plug this into my Windows7 box and mount the drive using HFSExplorer (http://www.catacombae.org/hfsexplorer/ - the mac version doesn't detect or load the drive at all). I'm then able to extract files - so I have a copy of everything EXCEPT the time machine backups. I don't have another drive large enough to make a disk image of the drive (which HFXExplorer can do). So I don't feel like a utility like DiskWarrior is neccesary since I don't know if it can repair the partitions/whatever to make the drive mount again.
I have since bought a new external hard disk and set it up as a new Time Machine backup drive and done a full backup to it. It's part of the backup drive set in case I can get this drive going again.
Is there anything else I should try before I format it again and/or put it in the rubbish bin.