Hello Paul,
You can achieve the same using AxCrypt app. You can use our portable version to secure your portable hard drives files/folders.
For portable(stand-alone) version, https://forum.axcrypt.net/download/ .
Please note: AxCrypt app encrypt file(s) with the .axx extension. AxCrypt encrypted files have their own file structure and AxCrypt app only can interpret and open those file(s). That’s the way AxCrypt was designed. Other than .axx extension files, can’t be opened by the AxCrypt app. No other application(s) can open the AxCrypt encrypted files.
If you are asking any other quires, you can feel free to contact our support via support@axcrypt.net .