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  #1  
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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Old 03-30-2019, 07:36 AM
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dnanian dnanian is offline
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I've never heard of Mojave being unable to boot from HFS+ volumes. Can you provide me with the "firmware change" command line they supplied?

To format back to HFS+, you need to "show all devices" in the View menu, select the drive hardware (above the volume) and then elick Erase. You'll have the choice of Mac OS Extended (Journaled) there. You don't have to use Terminal.

But, you can certainly leave it as HFS+ and restore to APFS. And APFS is certainly what you should be using with internal SSDs in 10.13 or later. You haven't indicated why you don't want APFS...
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Old 03-30-2019, 12:00 PM
derekw derekw is offline
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Originally Posted by dnanian View Post
I've never heard of Mojave being unable to boot from HFS+ volumes. Can you provide me with the "firmware change" command line they supplied?
Unfortunately I don't have the command line anymore. It was entered while the Mac was in Mojave and now the drive has been erased. Terminal "history" shows nothing.

This article might have related info. https://bombich.com/node/662 FYI, the external drive I used is a Seagate Backup+.

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Originally Posted by dnanian View Post
To format back to HFS+, you need to "show all devices" in the View menu, select the drive hardware (above the volume) and then elick Erase. You'll have the choice of Mac OS Extended (Journaled) there. You don't have to use Terminal.
I did not know your method. Apple tech support did not know how to reformat a APFS drive back to HFS+ either. When I googled, I found these results and followed their Terminal commands.
https://wp.me/pmPaT-cco
http://bit.ly/2HXMPNA
https://tek.io/2uCN2xQ

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Originally Posted by dnanian View Post
But, you can certainly leave it as HFS+ and restore to APFS. And APFS is certainly what you should be using with internal SSDs in 10.13 or later. You haven't indicated why you don't want APFS...
My Mac mini late-2014 only has hard disk (no SSD, no fusion). It was running High Sierra with HSF+, same for the clone backup. When I restore I did not know if it's safe to restore from a HSF+ clone to a APFS drive. When I asked Apple tech support about this they said I can try but they don't know if it's safe. I saw that SuperDuper "restore all files" function's first step is erase the drive. But I didn't know what format SuperDuper would use after the erase, so I felt safer to erase and format the drive myself to HFS+ in preparation for Superduper's restore.

Last edited by derekw; 03-30-2019 at 12:03 PM.
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Old 03-30-2019, 12:09 PM
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dnanian dnanian is offline
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There's definitely nothing in that article that would suggest a special terminal command is necessary to recognize an external device at Option+Power On... I'm quite familiar with everything in there. Would really like to understand what, exactly, they asked you to do, since it's totally news to me.

With regard to formatting, yeah, welcome to the new Disk Utility. Two steps forward, etc.

You definitely shouldn't use APFS for a regular HDD that you're going to be running from as your regular startup drive. But, for 10.14 and later, it's fine for Fusion and it's fine under all versions for SSDs. It's also preferred for backup drives, since you're not running from them all teh time...

Anyway, SD maintains the "major format" when erasing. It won't change HFS+ to APFS or vice versa. But you can also erase yourself and then Smart Update, or whatever.
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Old 03-30-2019, 03:01 PM
derekw derekw is offline
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Originally Posted by dnanian View Post
There's definitely nothing in that article that would suggest a special terminal command is necessary to recognize an external device at Option+Power On... I'm quite familiar with everything in there. Would really like to understand what, exactly, they asked you to do, since it's totally news to me.
I called Apple again but they can't find the command line they sent me yesterday. Meanwhile, I found this Apple support article about choosing startup disk which includes some command lines. I don't know if that's what I did though. https://support.apple.com/en-ca/HT202796

Extra info: There is a Option-Shift-Command-Period method mentioned in the article. I also tried that but it didn't work. I was using an Apple wireless keyboard so I don't know how reliable it is on these startup actions. The drive was not encrypted (no FileVault) and no firmware password was set.
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Old 03-30-2019, 03:24 PM
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dnanian dnanian is offline
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Yeah, that drive doesn't have an option rom, so that wouldn't be it.
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