One of our users made a startling discovery this week after upgrading to High Sierra. He had an HFS+ formatted 16TB RAID device, and had always intended to enable encryption on that volume. There’s no OS on it, so he simply right-clicked on the volume in the Finder and chose the option to encrypt it.
This is an easy way to enable encryption on a volume: plug in a password, verify, add a hint, done!
Oddly, though, [Carbon Copy Cloner], Disk Utility, and Terminal all agreed that his HFS+ volume was now an APFS Encrypted volume. Naturally he contacted AppleCare. “Not possible, says Apple”, he reported.
Yet again, Apple support doesn’t know what they’re talking about.
This is particularly worrisome. If you convert a disk to the new APFS (Apple File System), it won’t be readable from a computer that is not running on an APFS volume. So if you have a problem with your computer, you may not be able to access data on that disk. Also, this means you cannot encrypt a portable hard drive on an APFS computer if you plan to use that drive with a non-APFS computer (ie, one running macOS Sierra or earlier).
Source: Think twice before encrypting your HFS+ volumes on High Sierra | Carbon Copy Cloner | Bombich Software