It would be against the policy of this site to discuss any methods of unlocking/converting a commercial map from Garmin.
However, I'm surprised that you could not copy your backup to the card and get it to work. This kind of map is locked to the hardware ID number of the original card. I have always read that you can backup these maps to your computer and restore them in the event of a problem. Are you certain that you are copying all the original files to their original location on the card? You must use the same card the map was originally supplied on. I believe there will be three files: the .img file is the map itself and there would probably be two files used for copy protection, one with a .gma extension and possibly another with an .unl extension.
Aside from that, I think you're out of luck unfortunately.
It doesn't make much sense to purchase the Garmin topo maps on SD cards IMO. The DVD version is not locked and can be installed on any Garmin devices that you own, so a backup can be restored to any card. The download version will be locked to the original GPS, but it can be backed up and restored to any card. Garmin also allows you to download the map again at any time for 12 months.
Almost all maps on this site were created at 24k resolution. Not sure if it would be possible to fit the entire US into a single 24k topo map and stay within Garmin's spec. Garmin maps can only have a total of about 4000 segments - the smaller tiles that make up the whole map. If you purchase Garmin's own 24k maps, I know that you can't even come close to fitting the whole US before this limit is reached.