My original plan seemed straightforward: I have an iMac17,1 with an internal 2 TB Fusion Drive, and a spare USB3 750 GB SSD. I wanted to be able to start up in High Sierra, running on Apple’s new file system, APFS, but didn’t want to go the whole way to running only High Sierra. For the time being, until High Sierra is supported on Fusion Drives, I wanted to continue working in Sierra.
So why not install High Sierra on my external SSD, and then boot from that when I wanted to test or try something in High Sierra? Such dual-boot systems have been commonplace, and for a while one of my Mac Pros was triple-boot, with its four internal hard disks.
1: Let the installer do it
My first attempt was marred by Apple releasing the wrong High Sierra installer in the first place. Although it did install onto the SSD, there were several things wrong with it, including the fact that the drive had not been converted from HFS+ to APFS.
2: Let the right installer do it
Once I had downloaded the proper installer, I repeated the installation, only to discover that the external SSD had been left in HFS+ format. I had learned one lesson which Apple had not explained: the High Sierra installer will only convert internal SSDs to APFS. It leaves external SSDs in HFS+. There are no options.
3: Use a bootable installer
Using my installer app, the instructions here (which I had to hastily revise after Apple changed them following the release), and a 16 GB USB memory stick, I created a bootable High Sierra installer. I then restarted my iMac from that, and opened Disk Utility. I formatted the destination SSD in APFS (non-case-sensitive, unencrypted), then set the installer to work on it.
This works, but at the end of the install, when my iMac restarted, the SSD was nowhere to be seen, and I was back into Sierra.
On reflection, what I should have done then was to have restarted with the Option key held, to enter the boot menu, and selected the SSD from there, if it had decided to reappear.
Therefore, if you want to make yourself a dual-boot Mac, you should be able to use this process: format the SSD from Disk Utility on a bootable installer, then install from there.
4: Try the proper installer again
Now that I had my SSD in APFS, I tried running the High Sierra installer in Sierra, to install High Sierra onto it. The installer refused, demanding that the destination volume be in HFS+ Journalled format, not APFS. This was another important lesson: when run in Sierra, the High Sierra installer will not install to an external disk unless it is in HFS+ Journalled format.
5: Try the proper installer, running in High Sierra
At this point, I lost heart. Not entirely, but I resigned myself to running High Sierra on HFS+ for the time being. I therefore formatted the SSD in HFS+ Journalled, and ran the installer from Sierra to install High Sierra onto it. I now had a dual-boot system, even though neither drive was using APFS, which was much of the object of the exercise.
However, I had another USB3 SSD to hand, so what I then did was start up in High Sierra (HFS+), format the second SSD in APFS (non-case-sensitive, unencrypted), and then run the installer from High Sierra. Sure enough, it didn’t mention any need for HFS+ this time, and happily installed High Sierra onto the second SSD, on APFS.
Thus another way of making yourself a dual-boot Mac is to have two external SSDs. Install High Sierra on the first, using HFS+, then boot from that, format the second SSD in APFS, and run the installer to put High Sierra onto that.
6: There must be a catch
Before you rush off and try either of the successful methods above, my experience with this dual-boot system is that it is a complete disaster.
There are two problems which have already struck me. First, although Sierra can read from and write to this release version of APFS, this is not altogether reliable. A few copied files have errors, and a few attempts to copy files from APFS to HFS+ result in failure. Persistence pays off, but this is not a good way of working.
The worst problem is that of Time Machine backups. Although I have quickly turned Time Machine off after each High Sierra installation, each time that I have booted back into Sierra from High Sierra, and my normal Time Machine backups have kicked in again, there have been problems. On three occasions, these have resulted in deep event scans and large backups. Twice they have prompted Time Machine to run a full 1 TB backup. As a result of this, I have had to erase my backups altogether. They went back to 2012.
The most likely explanation is that High Sierra is damaging the hidden FSEvents information which Sierra’s Time Machine relies on to determine what to back up. Even that doesn’t explain the full backups, though.
So if you really want to run a dual-boot Mac, you’ll need to solve the problem with Sierra’s Time Machine backups. Or have a very large backup drive.