Uninstalling OS Lion, and transferring photos to MacBook if full restore needed I have, stupidly, upgraded to OS Lion on both my desktop and laptop.
I've now done a hard restore on my laptop, back to Snow Leopard, and reinstalled my photo software, which wouldn't work with Lion.
Now I need to remove Lion from my desktop, but must save 19,000 photos first, either by transferring onto my laptop (preferred) or onto my WD 2Tb hard drive, so I can reinstall after downgrade to Snow Leopard.

Crazy, I know, but I've never needed to do this before, so please be gentle with an upgrade novice.
Use Time Machine to backup your entire system to your WD drive.

Once finished, clean restore your machine back to Snow Leopard, re-install your applications and then perform a manual drag-and-drop restore from your Time Machine drive back to your clean machine.

This will avoid restoring any upgraded system files, preferences or other content Lion would have upgraded at point of upgrade.

I've recently had to perform the steps above for a family member after a misbehaving Lion upgrade (bad kernel extensions or preferences causing random shutdowns).