Just an update as I finally have my car. The 128gb flash drive didn't work. I then deleted just over half the files & it still didn't work. I then copied the 60gb of audio to a 64gb drive & only around half the files showed. I finally put 30gb of files onto a 32gb drive. Most but not all audio files showed up. I've come to the conclusion that the usb reader has a maximum number of files it can read & my 32gb drive has too many. I don't know if the maximum usb size will increase with a software update, but for anyone buying a 2016 XTR, I'd say don't bother buying a usb drive larger than 32gb.
Think I'll dig out my old ipod classic (which is a 160gb capacity) & see if that works.
So it seems I accidentally found the maximum, just due to my tightwad £5 budget when I was buying one.
You still get stupid amounts of capacity even at that size - probably a suitcase of CDs.
I don't know how many tracks I have on mine, but they're pretty high quality - so it's possible I hit the capacity of the drive before any limit on the number of tracks.
I've also found that I can play the music on my phone via Bluetooth. It seems quite buggy though - it saw one album with thousands of tracks then it all went a bit mad.
Just a quick update, the usb can read an ipod classic so that seems to be the only way to have a large number of audio tracks.