I have plugged a 1TB external storage and formatted it as internal drive. It works with media just fine.
In the apps section, when I tap "Storage used", for some apps it allows me to select the USB drive (with 880GB available), however it fails with "not enough storage space".
What does it mean? There is plenty of storage space in that drive, that's the reason I'm trying to move apps there...
Is your actual internal storage, not the attached drive, almost full? To move an app to the external drive it needs enough space on the internal drive to cache it. So if you are moving an app that takes up a gig of storage, you will need approximately that much free space on your internal memory to be able to move it.
Try clearing cache of your apps to free up space. Then move smaller apps first. If you still don't have enough room, try temporarily uninstalling a large app until you move the ones you want to move.