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I was upgrading my dad's 2013 Air yesterday and it got into a time-sapping install loop where it couldn't install 10.13 once I'd wiped the drive and reformatted to APFS. I had to enter internet recovery, wipe and reformat to HFS+, then restart the 10.13 install without wiping and reformatting.
I thought APFS was mandatory for all boot SSDs but nothing else. P'raps not.
mattioats
Well_is_it
Finally updated the work iMac (2013) to High Sierra.
It's booting from an external SSD in a USB 3 enclosure and everything works swimmingly.
Interestingly, though, it didn't change the filesystem to APFS on either the internal HDD or external SSD.
My personal MBP from 2011 did get APFS, however.
Maybe external drives and spinning disks don't get it and it's reserved for SATA/PCI attached SSDs.