When I click on an external USB disk in Finder's Sidebar it shows it is still encrypting (which is expected):
diskutil shows only
Encrypted: Yes in CLI and GUI.
Is there any way to check the current status of encryption (percentage complete)?
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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"