I have a USB-powered Western Digital hard drive that works with my N7 using an OTG cable. I was curious as to how heavy the battery usage would be when powering a spinning drive so I did a small test, playing the same movie from the hard drive and then from a flash drive. Heres what I saw (I charged the N7 between tests, usage number from battery monitor measure of charge level):

Stock 16GB N7
Play movie with MX Player Pro through Nexus Media Importer
Duration of 1 hour, screen at 100% brightness
Video: 832 x 352 1503 kbps h264 MP4

Results --

Sandisk Cruzer 64GB Thumb Drive, FAT32 -- 12% battery drain

Western Digital My Passport 500GB Portable USB 3.0 External Hard Drive, NTFS -- 23% battery drain