Physical USB diskette drive booting

With the raspberry pi I've gotten an image from the drive to boot, but trying to access the block device file (/dev/sda with full write/read access) I'm having trouble getting tiny to work with it, I tried linking it up a .img file but it knows it's not an image and doesn't work with it

"Cannot find image /dev/sda"

What would be the proper way to go about booting from an actual drive?