error backing up to encrypted HFS+ backup disk using Time Machine on High Sierra

I've recently upgraded my MacBook pro to High Sierra from Yosemite. I am trying to use a NEW external hard disk drive for Time Machine backups. I've formatted the drive to MACOS extended (journaled, encrypted), however when I mount the disk I am not asked for the password. When I try to set up Time machine backups with this drive by going to System Preferences > Time Machine > Select Disk > Use Disk, I get the following error:

"There was an error preparing the Time Machine backup disk XXX. This Core Storage logical volume is already decrypting."

I haven't been able to find much on the web about this error except a couple of websites stating that you shouldn't try to use encyption with Time Machine on High Sierra as your drive will be inadvertently converted to APFS. This has not happened in my case, Disk Utility shows that the format is MACOS extended, journaled, encypted.

Could anyone help me sort this out please?

I've never tried backing up to an encrypted HFS+ drive myself, so I'm not sure what would happen if I did, but from the message you are getting it sounds like TimeMachine doesn't want to use the encrypted drive, and is trying to decrypt it, which takes time. It is possible that the drive is currently decrypting and TimeMachine should start working once that is done. I answered another question about that here.

So you don't have to click the link, this is what I said there:

If you type the following command in Terminal you will get all sorts of info on all the corestorage volumes currently attached to your Mac, including the conversion status you are looking for:

diskutil cs list

There are more answers here.

One of my favorites is Nick Bedford's answer towards the bottom of that page that gives you just the status of each volume without all the other info.

diskutil cs list | grep -e "Conversion" -e "Volume Name"

If my theory is correct, once you have run that command if you scroll down a bit you should see:

    Encryption Status:       Unlocked
    Conversion Status:       Converting (backward)

which means the drive is being decrypted. If TimeMachine is not working because of the encryption then it should work again once that process is complete. If you have nothing on the drive and want to 'speed up' that process then you can reformat the drive as a normal HFS+ journaled partition, without the encryption turned on and then trying Time Machine then.

I'm not sure the encryption is the problem, but the error message seems to indicate that it is, so I am curious to see what the status is on the encryption and conversion status of your core storage partition, and if waiting for the decryption to finish/reformatting fixes the problem. Good luck!