I have a drive with two partitions. One is the new APFS with Mohave, the second is HFS with Sierra so that I can still run some old software.

Here's the problem. When I go into system preferences to set the startup drive to HFS/Sierra, no problem. Select, restart and there I am. However, when I want to go back the other way: system preferences to select the APFS volume to reboot into, Sierra is not able to see that volume because it can't read APFS. So, I can't select that as my startup drive.

So, how do I boot back? Anyone have some insights? What am I missing or is it not possible? Since they are on the same internal drive, simply unplugging the HFS to force the APFS startup is not possible.

Have you tried rebooting and holding down the option key? This is supposed to give you a list of available start-up drives to select from during the boot process. Presumably since the firmware of the computer can now read APFS, hopefully it'll see the APFS partition.

The option key is the only solution. From there, you use the startup manager.

The option key is the only solution. From there, you use the startup manager.

That's good to know. I hope I don't forget that when I finally upgrade my OSs.

The only down side is that I can almost never do that with a BT keyboard and have to drag out a wired version.

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APFS is awful. Why apple doesn’t allow non-Apple disk drives to boot is a real bad decision. My daughters 2015 MacBook somehow got ‘upgraded’ to Mojave’s and it won’t boot because it’s a third party sata SSD. Of course, she had file vault activated, so getting her files off and reinstalling high Sierra was a pita.

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RAMd®d
The option key is the only solution. From there, you use the startup manager.

That's good to know. I hope I don't forget that when I finally upgrade my OSs.

The only down side is that I can almost never do that with a BT keyboard and have to drag out a wired version.

Press the Option key on the BT keyboard immediately after the boot-chime. That's when BT initializes.

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Thanks. I've been away from my Mac for awhile this evening. I'll give it a try. I know I had trouble doing that before with the BT keyboard and just assumed you couldn't do it anymore. I assumed it was a part of Apple's security squeeze to keep someone unauthorized from booting into another volume.

I do know that used to work in the older Mac OS.