In reply to rallymania:
> ... you could also transfer the files from your SD card onto the device, and then plug in a USB flash drive to back up the tablet copies too. again i don't think the hard drive based USB drives will work, but the solid state (non-encrypted) flash drives should be fine to use
Same in my experience. Flash drives are fine, but the tablet fails to provide enough power to run my hard drive (old laptop drive in a caddy) and it starts-stops-starts-stops continuously.
I've never tried a powered hard drive, mostly as those aren't much use in the circumstances where I need this sort of file transfer.
Eye-Fi card and app could also be a viable alternative to the OTG cable if just shooting jpeg, though this would only store the shot on the tablet. They would need to be backed up onto another media such as a flash drive.
SLR with 2 slots, one with SD/CF and one with Eye-Fi might work - still only jpeg though.
> depending on your camera, you may also be able to use the USB OTG connector and your camera's USB cable to convert the tablet into a screen / controller for the camera itself. (i have canon 600d and the dslr-controller app on my tablet which works a treat)
DslrDashboard on my nexus works well with my D5100. Not tried dslr-controller, though probably will give it a whirl when I have more time to play.