How do I save iBooks pdfs to an external hard drive?
I'm not going to post the church boy answer, but if you jailbreak your device, you will have full access to the file system.

There is probably a legitimate way to do this without jailbreaking, but having jailbroken devices myself, I would simply enable SSH (the root password is alpine, which you should change immediately) and either rsync or scp the files to your external hard drive volume (which in my case is mounted on a Mac Mini (also running sshd).  The hard drive will be mounted in /Volumes.

I forget the exact path to your Camera Roll, but the main directory you are looking for containing the photos and videos is DCIM.

If your external drive is a Windows computer, then there are also tools in Cydia that make file sharing with Windows a breeze.  Or if you're like me and have to run Cygwin with sshd on all of your Windows boxes, the same rules as above would apply, but the path to the external drive would be /cygdrive/[drive letter].

If you are on a Mac and enable SSH to your iPad, just do the following in Terminal on your Mac (if it is a Mac):

cd /Volumes/[external drive]/[folder to store images]
scp root@[iPad IP address]:/path/to/DCIM/* .


It would be a good idea to use screen for this job so that it doesn't fail from a broken pipe in case you have a lot of images.  If you want to go that route, you could do something like this screen -LS image.copy which would put you in a new shell session.  Then run the commands above, and detach from the session with Ctrl+A-D.  This lets the process run in the background.  If you want to come back later and check the progress, run screen -x image.copy.

If you are going to do something like this, you should make sure that you are hooked up to power and don't have auto-lock set, or else it will end your session prematurely.

This may be a horrible answer, but I always default to command line, because it never fails me.