As breedBenR says there is no magic formula to this I have 3 different makes all can take an external power lead there are two 500gb and one 320gb only one of the 500's works without power. Also the note about fat32 is important as android does not natively support ntfs - I really hope this comes in a future release, as today the apps that do allow this require root and I won't invalidate my warranty .
I agree with breedBenR. As long as it is in FAT32 format and have a good quality USB cable to connect to your tablet, it will work. Though other hard drives eat a lot of power which can only make it impossible. It won't hurt to try though.