A friend who's 60-odd km away emailed me with an odd problem: the SSD from his failed 2011 MBP when installed in an external case, wouldn't mount on a 2012 Retina MBP borrowed from his son.
Yet this same drive would mount on his NAS. He wanted to get some 300 GB of stuff off it but couldn't work out how to do it. A different ext drive setup (3.5-inch in case) mounted OK.
I suggested he put the SSD back in the MBP & restart it in target disk mode, connecting the two Macs via a Thunderbolt cable.
He said he'd give it a go but wasn't prepared to buy a $60 just to do that.
I suggested he could borrow one from me, if he was prepared for a 120 km round trip as my car was out of action. He asked for my address & said he'd be there by 10 am. He was 5 minutes late, as it turned out.
I've heard back, after a few attempts the old MBP in target disk mode had mounted on the desktop of the new one & he was transferring the stuff from it.
This is just one of the reasons I insist on at 2 clones & at least one TM backup - there'll always be a copy that's only a few days old if recovery is needed.