@ neil
I tried your exact copydir command as per your pack. (LDR.BIN on SD Card Method) on Nuvi 1300 that works. I did it last night:

copydir,0:/GPX,1:/GPX
reboot

What happened is that it tried to dump the entire nuvi inside the folder GPX.
I stopped it halfway.

This would explain why the OP has a 1.13 gig folder called GPX.
The current.gpx will be there though, Mission accomplished.

I have previously tried it on Nuvi 3490 (Same PROBLEM)
If you have some time would you try it again please, just individual folder using the copydir command.