Short story version: use the CIPHER command.
I had a 3-2-1 backup fail on me recently."
“21,684 files could not be copied”.
In technical terms, the second computer did not have the EFS decryption key. The easy fix was to remove the EFS encryption via the CIPHER command:
cipher /a /d /s:<directory name>
/ais all files, including subdirectories
/s:is the directory name.
To bulk encrypt, you just need to replace the
cipher /a /e /s:<directory name>
Note: the cipher command will fail if the files have the READ-ONLY flag set (ie. remove the flag first)