To make sure I'm understanding you correctly, you have VitaShell running with nothing connected to the PS TV via USB, click the mount uma0: option in VitaShell, a message prompt in the center of the screen pops up saying to connect your USB thing, you plug in the 1TB or 3TB drives (which are in powered USB. There's an option you can enable in Settings > System called 'USB Power Supply', it's enabled by default. On the Vita, the 'Content Manager' icon should be flashing its little head off.
Vitashell usb error I believe it's supposed to give you the ability to charge your Vita while it's plugged into your PC via the USB cable.
