Immich on usb hdd

Hi team,

how is it possible to use an external hdd in immich?
I can attach an external device in the immich settings.
But if i try to attach the folder “/mnt/usbdrive/immich” it cannot find it.
Guess it is because the containers behind are not able to attach the folder.

Do you have an idea?
The internal hdd with umbrel installed is far too small.

Thanks!