You can use udev to set up hardware-specific devices. Simply add a rule
to have it symlink the device to a particular /dev entry. It's probably better however to create and use a dedicated device with a memorable name like "backup_disk_a" than to try to mess with the traditional sd* devices.
And actually, udev has already set up some links for you. You can find permanent symlinks to each block device in the /dev/disk/by-whatever directories. You can use those to access the device you want, no matter what the /dev/sd* number is.