SD slot suddenly not working in 765T.

If you purchased the map on an SD card from Garmin it is locked to that card and that card only. It cannot be copied to another device.

I should also add that both cards tested to work nomally in a card reader connected to my PC. This worked before. I cannot think of a reason why SD card is not accesible anymore by 765T. I wonder if firmware upgrade to 4.00 has anything to do with it.

I have a 755t with software 4.0 and my SD slot works fine. I don't have maps, but I do have JPG's and MP3's on my SD card and have had no problems with 4.0. You might try a hard reset as that often will resolve strange problems after an update. If that doesn't help, I would suspect a hardware problem.

I found this on Garmin Support Search.

Why doesn't my device recognise my Plug and Play Card?

Answer:
If you are having problems accessing the data on a Garmin Plug and Play card of mapping data on your Nuvi try completing the following steps,

Make sure the Contact pins on the card are Clean. If there is any dirt on these it may prevent the card being properly read by the device.

Make sure the SD card reader is Clear from any substances such as Dust. It may be best to gently Blow into the slot to remove such substances.

Perform a Master Reset without the card inserted:
Once this has been done check the Map Info on your device to ensure that the new mapping is recognised.

From the front screen on the Nuvi touch the option for Tools/Settings. (The first step will depend on the unit you have)
Touch the option for Settings (Only if you touched Tools on the first screen)
Touch the option for Maps
Touch the button marked Map Info you should now see a list of all the maps currently recognized by the Nuvi.
Verify that the Maps contained within the card are listed

Master reset did not resolve the problem. I have previously tried cleaning the contacts on the SD cards. Looks like the SD slot is just dead. The GPS is functioning normally otherwise.

I have noted similar behavior on my 765T but it has started working again. Rather than "master reset", remove the card try a hard reset by sliding and holding the power button to the left and force it to reboot. While it is powered on and finished booting, try inserting the card while in media player, or if a map card, in the map list display and see if it comes back. I have suspected there may be something in the newest 4.00 firmware that gets a little hung up while monitoring the SD card port.

I had the same thing happen to me. I had to send it in for an exchange under warranty.