I know connecting an external hard disk to the USB port is unsupported, however I noticed a couple of things that might indicate a bug/code cleanup elsewhere.
When a hard disk is connected to the USB port (unmounted) and you go to the status page, only one of the internal hard drives are shown. In my case, the right one. This happened in the v7 firmware too, but in the v7 firmware the USB disk details would be shown in the printer status area of the staus page.
New to v8 I think, in the RAID tab the USB disk details are shown under the "Audo rebuild configuration" heading. Seems a little odd that code exists to spit that information out there.