Can I downgrade to High sierra from Mojave easily

levmc

macrumors 6502
Original poster
Jan 18, 2019
281
8
I haven't made a backup of the computer though when I was on High Sierra. Is it still possible?
 

Partron22

macrumors 68030
Apr 13, 2011
2,565
756
Yes
Mojave update will modify your firmware. This is not undoable. You will be able to go back to High Sierra, but booting different OS's off external drives will never again be so easy once your internal drive has seen Mojave. There is no way to prevent this via making backups.
 

chrfr

macrumors G3
Jul 11, 2009
9,541
3,426
Mojave update will modify your firmware. This is not undoable. You will be able to go back to High Sierra, but booting different OS's off external drives will never again be so easy once your internal drive has seen Mojave. There is no way to prevent this via making backups.
The firmware updates have no relationship to being able to run older operating systems. In fact, installing security updates for High Sierra update computers to the same firmware versions as included with Mojave and its security updates.
 

Partron22

macrumors 68030
Apr 13, 2011
2,565
756
Yes
The firmware updates have no relationship to being able to run older operating syystems...
That's not been my experience. Ever since I tried Mojave on an external HD, I have been unable to boot to anything except internal HS, external HS or external HS or Mojave, with my external HS SSD TURNED off. It's very weird behavior, but that's what I see. Perhaps I am the only one, but the behaviour is consistent with firmware problems, or a partial mod of drive for APFS comparability. The second external SSD must be turned Off to get a boot to the first ext SSD. Again, I may be the only one experiencing this trouble, but it it is a huge PITA.
 

chrfr

macrumors G3
Jul 11, 2009
9,541
3,426
That's not been my experience. Ever since I tried Mojave on an external HD, I have been unable to boot to anything except internal HS, external HS or external HS or Mojave, with my external HS SSD TURNED off. It's very weird behavior, but that's what I see. Perhaps I am the only one, but the behaviour is consistent with firmware problems, or a partial mod of drive for APFS comparability. The second external SSD must be turned Off to get a boot to the first ext SSD. Again, I may be the only one experiencing this trouble, but it it is a huge PITA.
It's definitely just you.
 

Fishrrman

macrumors Core
Feb 20, 2009
19,390
6,701
"Mojave update will modify your firmware. This is not undoable. You will be able to go back to High Sierra, but booting different OS's off external drives will never again be so easy once your internal drive has seen Mojave."

This is nonsense.
Disregard it.
I've tinkered with Mojave on my old 2012 Mini, and then went back to Low Sierra.
There are NO PROBLEMS WHATSOEVER booting from older versions of the OS, so long as you have a good drive and good OS install from which to boot.

OP:
If you wish to "go back", it's going to be more difficult if you don't have either a cloned backup (created with CarbonCopyCloner or SuperDuper) or at least a TM backup.

It can be done, but it will probably be more "work".

First, you need to get some kind of backup of your apps, personal data, etc.
I recommend a bootable cloned backup using either CCC or SD.
Then, boot from the backup and ERASE the internal drive.
Then, re-install High Sierra (you may have to create a bootable USB flash drive).
Then, you could -try- setup assistant to bring over data from your backup, but be aware that setup assistant (and migration assistant) generally only work "one-way" (from older to newer), and you may not be able to migrate "downwards" (at least by using Apple's tools).
That means "a manual migration" where you pick-and-choose stuff in the finder and then put it back "a little at a time" -- one or two days' work, at least.

See what happens because you DIDN'T create that bootable cloned backup BEFORE you upgraded?
 
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Partron22

macrumors 68030
Apr 13, 2011
2,565
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Yes
It's definitely just you.
Well, I guess that means I'll have to play with it some more. It's definitely a weird problem, and persistent.
I expect there's at least a few others who've experienced it. But if not many, that means there's likely a solution. I'd be quite happy to find it.
 

chrfr

macrumors G3
Jul 11, 2009
9,541
3,426
Well, I guess that means I'll have to play with it some more. It's definitely a weird problem, and persistent.
I expect there's at least a few others who've experienced it. But if not many, that means there's likely a solution. I'd be quite happy to find it.
Perhaps it's an issue with your external drive that has nothing to do with operating systems. Does it have up to date firmware, have you tried different cables, etc.?
 
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Partron22

macrumors 68030
Apr 13, 2011
2,565
756
Yes
Yeah, chrfr, That's the game I'm about to embark on. Can't say as I'll enjoy it though. This type of troubles tend to be well hridden and tedious to find. At least now I know it's a problem for me, but not others.
 
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