I did this using the micro USB port on the Pi using an adapter.
I pre-configured my USB stick formatting FAT32 as per normal and creating a folder on the root called 'retropie'.
This worked fine and the Pi created the relevant folders on the stick.
My build is now complete and ive attempted to load more roms on via the USB port on my GBZ but this doesn't work.
The Pi will not pull any new roms.
This USB port works fine with gamepads, keyboards, Bluetooth keyboards etc so confident its working OK.
Have re-formatted the same memory stick FAT32 and re-created a 'retropie' folder.
When I put the USB stick in, it flashes as normal for a few seconds then stops. When I put this back into my computer no additional folders have been created. Ive also tried other USB sticks.
Any ideas?
Havent re-tested plugging directly into the Pi's micro USB again but assumed this should work as above??

Help appreciated.