You can achieve the same using AxCrypt app. You can be connected to the external USB/hard disk/drive to your system/laptop, then encrypt your files using AxCrypt app.
AxCrypt has a feature called Secured Folder which provides the convenience to the users, to secure more than one files in a single operation. You can encrypt the Drive/disk folder/file(S) using AxCrypt Secured folder features.
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.