The Nuvi 780 is pretty ancient by consumer electronic standards, it was introduced in 2007.

I'm almost certain it used the really slow USB 1.0 interface. Garmin has been very slow to update USB on their devices, I don't think they offered USB 2.0 until a few years later, and then only on the high end models.
So it makes no difference what kind of USB port you have on your computer, it can't send data any faster than the Nuvi is capable of receiving. Transfering large files over USB is always painful.
Note that with your device, the file must be in a folder named Garmin and it must be named gmapsupp.img in order for the map to work. There are other reasons why it might not work also. For example, Garmin's commercial products are copy protected an will only work on the device for which they were purchased.