IIRC it's technically possible to attach an external harddrive to a Raspberry Pi if it has its own power supply.

I seem to remember doing a botch where I took a USB hard disk drive that was supposed to get its power from the PC through the cable and rerouted the power over USB lines to a dedicated power brick.

My memory says I carefully removed a section of mantle in the middle of the drive's USB cable, cut the power carrying lines but leaving the data lines intact, cut one end of a different USB cable, connected the power lines of that with the cut power lines of the drive's cable (only on the drive side, obviously), put the intact end of the second cable into a USB charge plug, and connected the drive and RPi as if the RPi were a regular PC.

I'm pretty sure it worked.