I shared a little tip about backing up to a USB drive a while back, and after a bit of tinkering, I have a new method which uses rsync to speed things up a bit, and as the name suggests, keep things in sync rather than just blindly copying.

The other thing that I'm suggesting, is that you mount/unmount the USB drive between backups, mainly to protect the filesystem if it's mounted when the power goes out, or if you have to grab it in a hurry.

So here is what I suggest setting up as a cronjob (replace sdb1 with your own usb drive info):

mount /dev/sdb1 /mnt/backup
rsync -vrlptg --del /path/to/photos/ /mnt/backup/photos
umount /mnt/backup/

Important to note, I have used the --del option here, which means files deleted from the source location, will also be deleted on the backup drive. Since I use my backup drive more as a "mirror" of the data, rather than an archive - this works well for me, your situation may be different.

A nice way of going about protecting your data, is to use 2 (or more) drives and alternate them once a week, this script will keep whichever one you have plugged in up-to-date.