Which encryption algorithm do external hard drives use?
They are encrypted with the AES encryption algorithm.
After you have encrypted the external disk, you can open Terminal and run this command to check the encryption:
diskutil cs list

It will show the disk, and then something like this in the output:

    Encryption Type:         AES-XTS


Here AES-XTS means that the main encryption algorithm is AES, and that the XTS block cipher mode is used.
The XTS block cipher mode is used primarily to prevent that identical blocks of data (for example &quot;never used space&quot;) encrypts into the same encrypted value. This is done by actually having 2 AES keys and a process where the data is continuously &quot;tweaked&quot; specifically to avoid such a problem.
If you have chosen to format the drive as APFS and not HFS+, then it won't show up when running the above mentioned command. You can check if that is the case by running:
diskutil apfs list

This will show the disk as being APFS formatted. However, it won't show you which encryption type is being used on the drive.