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Old 03-30-2019, 12:10 AM
derekw derekw is offline
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Downgrading from Mojave to High Sierra ... lessons learned

Two surprises: Can’t boot up from external backup; Can’t reformat drive from APFS to HSF+.

Before upgrading to Mojave, I made a full disk clone of the High Sierra HD (2014 Mac mini, no T2 security chip) to an external drive. I verified the external drive and see it boot fine. After I upgraded to Mojave I immediately noticed something not working right(1), so I decided to downgrade back to High Sierra using the clone. But I found 2 major obstacles.

Booting up using the external clone drive does not work. Restart with Option key shows only the Mojave internal volume, the High Sierra external clone does not show. So I let it boot into Mojave and go to System Preferences/Startup Disk, and I was able to see the external drive there. So I choose the external drive and click restart. But it boot straight into the Mojave drive, bypassing the external drive.

What I learned from Apple tech support is that during the upgrade to Mojave the firmware on the Mac (may be the BIOS?) was changed and the new firmware does not recognize external HFS+ volume. So during the early stage of booting up, before any part of the macOS is loaded, a Mojave machine cannot recognize a bootable HSF+ volume. Once booted up Mojave can work with HFS+ volumes fine, but not during the early stage of booting up. Apple tech support send me a Terminal command line to change the firmware back to something that can recognize HSF+ volume. And then I am able to boot from the clone drive using the Option key.

Once booted using the external High Sierra clone, I want to use SuperDuper to restore all files back to the internal drive. Before that I launch Disk Utility to erase the internal drive, which by then is formated to APFS by Mojave. But it does not work. Disk Utility only offers to format the drive to APFS, no HSF+ format (“Mac OS Extended, Journaled”) is offered. My High Sierra clone is HSF+, so I want to restore to a drive which is also HSF+. Apple tech support couldn’t help me there. They said they recommend APFS and I can try restoring the clone onto the APFS drive but they can’t guarrantee it will work without problems. I have to Google and found that to reformat a APFS drive to HFS+ you need to use multiple command line in Terminal. Not that easy, but I got it working. After that I use SuperDuper to restore all files back to the internal drive. Boot up fine, but Disk Utility find some corruption (even though the clone shows no such corruption). I have to boot up from an external drive to repair the internal volume. Disk Utility finds “Volume header needs minor repair” and it repairs it fine. After this there is no problem. But I learn some lessons, it’s not easy to downgrade from Mojave to High Sierra.

Footnote 1: What did not work right for me in Mojave is that iTunes got upgraded to a version (12.9) not available to earlier macOS. My other Macs at home with High Sierra and El Capitan use an older version of iTunes (12.8) and cannot be upgraded to 12.9 and the older version of iTunes cannot access the iTunes folder that has been touched by Mojave, so mounting my music from my other Macs does not work anymore.
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