Any HDD will work. Powered drives are better since the Wii U has low usb power. Flash Drives work. USB Powered Iomega Zip 250 drives also work lol
SCSI HDD would work if there was a decent usb converter......
I use a 128 GB Flash drive
Smash bros is the only game that uses it i think. you can load games off of a sd card but only if you place it in a usb converter. then it acts as a usb flash drive.