@ lolypop000 and tiger555 thanks so much for your help and suggestions.
Here's the results of my tests.


"On SD card in folder Garmin rename your map to gmapsup1.img and try if nuvi see map."

Tried this, didn't work.


"You can also for testing, rename your old map to gmapsupp00.img with this you will disable old map and try if your nuvi see new map from SD."

Tried this, didn't work. Also tried it with the old map renamed gmapsupp00.img and SD map changed back to gmapsupp.img


"I'd say check your spelling by right clicking the gmasupp.img on SD and selecting properties from the menu."

Checked this and it is correct. Garmin\gmapsupp.img


"Another quick way to check the SD is to put any JPG on the SD and you should be able to see it with the 755 photo viewer"

Tried this and was able to view the JPG on the device.


"Out of curiousity... what is the firmware version of the 755 update installed ?"

I have V4.5 also.


"If you have a .unl or .gma files in the garmin folder of the 755 you might try temporarly removing/renaming them"

I do have them, renamed to: gmapprom.unl.bak and gmapprom.gma.bak.
Didn't work.


"are you using the JCV and 3D buildings?"

There is a folder on the device named JCV, there are two files in it, D1279020A.sum and D3178010A.JCV
Not sure about 3D buildings but there is a file named GMAP3D.IMG and one named gmap3d.sum

"The following SD setup should definitely work for a 755t
sd:\garmin\gmapsupp.img
sd:\garmin\gmap3d1.img <---- must also be unlocked to use
sd:\garmin\JCV\D1279220A.JCV "

I have Garmin\gmapsupp.img but not the other two. Are they required?

This is so very strange.