The borked SSD is in a ext usb housing.
Connected to a Mac mini with High Sierra.
That Mac mini is booting from an external SSD drive. The two internals are clones of the external. I haven’t gotten time to installing the SSD internally. All a byproduct of being hit with Too Many Corpses on 10.13.4 update.
Performance is dandy.
Q: How do you suggest making a disk image of the borked HS install. Are you suggesting using something like Carbon copy clone?
I wonder if there is something with your hardware or your setup that is contributing to the problems you're having.
CCC copies files. Making a disk image makes a copy byte-for-byte of your SSD. There are instructions on how to do this in:
https://www.cyberciti.biz/faq/how-to-create-disk-image-on-mac-os-x-with-dd-command/
Some notes on using dd:
You're creating a file of what is on the SSD. It might take a while. I did a test on an internal 240GB SSD on my Mini - just 650MB worth - and if the performance held up, it would have taken under 5 minutes for the 240GB SSD. Just be prepared for it to take some time. I had to do this recently on a 64GB flash drive and it took about an hour - so be patient.
1) In the diskutil listing, to reiterate what is said in the article - you're looking for the diskX that corresponds to your problem SSD - not diskXsY.
2) In step 2, you can eject the disk from Finder - it does the same thing.
3) In step 3, instead of diskX, you can use rdiskX - this should be faster. For "of=", if you want the output to be on another external disk, it would be: of=/Volumes/mydisk/directory/filename where mydisk is your external disk name as it appears in the finder. If you just use a file name, the file will be saved to your home directory. If there's a space in your directory or file name, you need to enclose the full path in double-quotes. I would use "bs=64k" as noted in the 3rd example. There's a section that says "You can create compressed disk image as follows". I would use this because if you don't use compression, the image will take 1TB for your 1TB SSD. Using compression can significantly reduce the size of the disk image file.
There's information on how to use Disk Utility to make the image. For me, I just prefer to use the command line for making a disk image. I've also seen posts from people saying that the disk/folder imaging in Disk Utility doesn't quite work - I haven't investigated this to know that's all about. It might be that opening an image for an APFS disk (being able to mount the image) doesn't quite work - I'm not sure.
So again, the idea of doing this is that you have an exact copy of what's on your SSD for the purpose of restoring it if whatever action you take to fix the SSD makes it worse. There's information in the article on how to restore to the SSD - if you use compression to make the image, you have to use the example which has the "gunzip" command to restore it to the SSD. Because you have multiple disks, I would redo the first "diskutil" list command to make sure you have the right disk number. It may change between the time you do the image and the time you restore to the SSD.