I don’t know if we are talking about the same “cases” I mean the hot swap ones, you put in 3x5.25" bays in your pc with sata backplate, and no usb external cases, those are crap I had several of those, bad iop throughput, and huge bottleneck.
Yeah, we awaiting your 70TB 
Seriously, try to restore original partition table with gdisk, may be testdisk hasn’t touched backup GPT table.
Launch gdisk on your damaged drive, it will analyze disk and may notify you that there are good backup version.
If that’s your case, then use r to enter recovery and transformation mode, then use b to rebuild your main GPT from your backup GPT. Use p to print the partition table and, if it looks correct, use w to write the changes to disk and exit gdisk. After that you need to reboot or reattach the disk.
I mean this one: ST-5255 - Inter-Tech Elektronik Handels GmbH
So not the same like yours but similar I guess.
Fingers Crossed 
Recovering the Partition Table unfortunately did not work, testdisk wrote over everyting. But the “transplantation” of the partition table with a new UUID worked unnexpected well, the toshiba got recognized instantly, like nothing happened. The On/Off WD seem to work. I have to say, I now added all sata cables fromt the PSU to the Bays, until now I hat one wire with 3 Sata connnectors, each had a y-cable, to feed all 3 Bays with 2 sockets each, maybe it was too much for one powerline. Unfortunately as I was trying to resilver the “defective” HDD, yesterday, the new Drive now is faulted which doesn’t matter, since it’s a pretty empty one (I can use the old one). For looks I will try to remove it later.
3 Likes
Yes something like that.
Much better. It’s possible/likely that one of these “y-cables” caused your whole problem. Many low quality examples exist, very difficult to obtain good quality ones.
You can do a lot to a ZFS pool before you kill it.
When I create an array, I partition the disks myself prior to creating pool/vdev, then feed ZFS the partition instead of whole device.
The second small partition openZFS has created on your drives is redundant and not required, can safely be deleleted - you can extend the ZFS partition if you really want that space…..
I don’t know if testdisk did a bad job at recovering the partition table with just one big partition and 3M left instead of 9M (8M spare partition + 1M (If a new drive is slightly smaller I think). The drive wouldn’t be recognized anymore. I’m lucky copying the partition table worked, so I don’t care for the additional 8M space 