@CaptainfingerbangI had the same issue and found the solution.
When using ARPL, you need to set the correct number of max drives when you configure the ARPL.
when not setting this number manually, if you don't use enough internal discs, all additional USB drives will be seen as internal discs in their place. Like drives 21, 22 and so on.
Setting the max number of drives correctly, will fix the problem.
Number: ARPL disc + your internal discs.
Like: 1 + 2 internal discs mean set it to max 3 discs.
Then, all usb discs attached, will normally be seens as external discs.