MTP Access

[AC Question]

I'm having a problem using MTP to transfer media files.

MotoE2(4G-LTE) XT1524, Android 6.0.

Phone shows:
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Device Storage
7.31GB
Total used of 15.28GB
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Android OS
3.00GB
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Internal storage
4.29GB used of 5.00GB
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Samsung SD card
16.89MB used of 7.28GB
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When I connect the phone to my PC using a USB cable and switch to MTP connection, the phone appears as expected. However, neither the SD card itself nor its circa 7GB of free space appear, only 'Internal storage'. I cannot transfer media files beyond filling up the 5GB of 'Internal Storage'. This behaviour is recent.

What's gone wrong and how do I put it right?