AxCrypt is a file encryption software not a disk/drive encryption software. But you can encrypt the disk/drive folder/file(S) using AxCrypt Secured folder features.
You can be connected to the external hard disk/drive to your system/laptop, then encrypt your files using the AxCrypt app.
AxCrypt Secured Folder:
AxCrypt secures files, individually. However, you can designate folders as ‘secured’. The folders will be monitored for new files, and AxCrypt will secure all new files with a single click, or when you sign out.
You can also select multiple files, including all files in a folder, to be secured in a single operation. They still remain individual files, and the folder itself is not secured, just the files in it.
AxCrypt 2 uses a more robust method for the automatic re-encryption, which sometimes means you as the user have to tell AxCrypt to do it. You’ll see this by the “broom” icon becoming red. This indicates there’s something in need of “clean up”, i.e. re-encryption.