Default MTP connection not showing lots of files on PC

Is anyone else getting this problem whereby the MTP connection to PC does not show a large percentage of the files stored?
On the phone, all media shows as expected in associated apps, root explorer sees all files.

The problem for me appears to be on my external sd card (its a Samsung 64Gb, formatted to exFat)
I was wondering if there is a way to force a refresh on the PC side?
Barring that, I think the only option I have is to back up the card and re-format it.(which will take me a couple of hours!)

Device: Samsung GT-I9505
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