Finder’s Unwanted Conversions to APFS
Take any HFS+ formatted volume that does not have an installation of macOS on it (that part is key), right-click on the volume in the Finder and choose the option to encrypt it. Rather than simply converting the volume to a CoreStorage Encrypted volume and keeping the HFS+ format, macOS converts the volume to APFS with no warning, and then enables encryption.
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APFS Encrypted volumes aren’t backwards compatible at all, so if you attach that converted disk to a Mac running an older version of the OS, you’re greeted with this heart-attack-inducing error message[…]
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But if I disable encryption on my APFS volume, the APFS part sticks – it’s not possible to convert an APFS volume back to HFS+ without erasing it.
Also importantly, if this is a spinning disk, converting it to APFS will make it much slower.