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I was having similar trouble with my Montana 650t.
I successfully downloaded a bunch of maps to BaseCamp, where they showed up, no problem.
I then used MapInstall to put them onto my 650t, which also had a 64GB microSD card installed.
MapInstall indicated all the maps had been installed to the 650t, but the 650t did not show them all on the list of enabled/disabled maps, only some of the maps were listed.

I was curious so I removed the card and I tried to read the microSD card on my PC.
My PC recognized the card but no data showed up on the card despite what MapInstall thought. My PC also indicated the card had to be formatted before it was used.
So...I used the PC to format the card to FAT-32 and at the same time it deleted any existing data (maps).
Then I put the newly formatted card back into the 650t and connected to my PC, opened MapInstall and reloaded all the maps I had deleted.
When it was finished all maps were now listed on the 650t !!! EXCELLENT!!
So if you have this problem, another thing to check is that your data card has been formatted before you try to load maps onto it.
I hope this helps some folks!

Edit: For the 64GB card I should have perhaps used format type: exFAT instead of FAT-32