"What I'd like to know is, is there any way I can downgrade back to HFS+ and still be able to use my Time Machine back up, or is that pretty much useless now?"
You can downgrade, but it's NOT going to be an "easy" procedure, because you used Time Machine, instead of creating a bootable cloned backup (which you should have done by using CarbonCopyCloner or SuperDuper).
Important question:
Have you been using TM since you upgraded to High Sierra?
If the answer is "yes", I'm wondering if you can still use TM to restore data to Low Sierra?
I can't answer myself because I have never used Time Machine and never will.
Here's what I suggest you do (and it's going to be a lot of work):
First:
- get an external drive large enough to hold the contents of your current installation
- use Disk Utility to erase the external drive BUT MAKE SURE that you erase it to Mac OS extended with journaling enabled (DO NOT erase to APFS)
- download CarbonCopyCloner (it's FREE to download and use for 30 days):
http://www.bombich.com/download.html
- use CCC to clone your internal drive to the external drive. AGAIN, if CCC offers to do an APFS clone, decline that and do HFS+ instead. (I'm not even sure if that's possible, I won't touch APFS with a 10-foot pole!)
- once the cloned backup is done, disconnect the drive and set it aside for now.
Next:
You're going to have to create a bootable USB flashdrive installer to ERASE the internal drive. Here's what to do:
- you need a USB flashdrive 8gb or 16gb
- ERASE the flashdrive to Mac OS extended with journaling enabled (GUID partition format)
- download the FREE little utility called Boot Buddy from here:
https://sqwarq.com/boot-buddy/
- you need a copy of the LOW Sierra installer app
- use Boot Buddy to create the flashdrive installer -- takes only a few clicks of the mouse. It will take some time to create it afterwards.
Next:
Install a copy of LOW Sierra:
- Boot from the USB flashdrive (reboot, IMMEDIATELY hold down the option key CONTINUOUSLY until the startup manager appears, pick the flashdrive with the pointer and hit return)
- it will open to the installer, but DON'T run it yet.
- quit the installer and open Disk Utility. You should be able to see the physical hard drive at the top of the list.
- ERASE the internal drive to Mac OS extended with journaling enabled. NUKE IT BACK TO ZERO.
- quit Disk Utility and re-open the OS installer
- let the installer do its thing -- it may go through one or more reboots, and take a while
- when it's done, you will be sent to the setup assistant.
Next:
- you can TRY to go through the installer sequence to the point where it asks you if you wish to migrate from an existing drive.
- at this point I would connect the CCC cloned backup, and TRY to bring over at least my old account and data settings.
- THIS MAY NOT WORK, because you're trying to go from "newer to older" insofar as OS versions are concerned.
- IF THIS DOES NOT WORK, I suggest you just create a NEW account for yourself, with your regular username and password.
Next:
HOW TO MANUALLY MIGRATE DATA
- Connect cloned backup drive, DO NOT open the drive icon yet
- Click ONE TIME on the drive's icon to select it
- Type "command-i" (eye) to bring up get info
- At the bottom, click the lock icon and enter your (new) password
- in "sharing and permissions" put a check into "ignore ownership on this volume"
- Close get info
THIS "WORKS AROUND" PERMISSIONS PROBLEMS THAT MIGHT OCCUR BETWEEN THE OLD ACCOUNT AND THE NEW ONE
Next:
Copying data:
- Be aware that you CANNOT COPY the "main sub-folders" named documents, movies, music, pictures, etc. in your old home folder.
- However, you can OPEN these folders, and then copy WHAT'S INSIDE them without problems
- So, you have to begin "working your way through" your old account folders, opening them, selecting what you wish to move, and then copying the selected folders/files into the new folders in your new account. It's going to take some time. KEEP WRITTEN NOTES.
Slowly, you can get this done.
Figure on at least one full spare day off to get the job done. Perhaps longer.
You wanted to know how to "get back"
This is "how".
Next time, do a fully bootable cloned backup using either CarbonCopyCloner or SuperDuper.
If you had one of those, getting back would be easy by comparison...
