RB6 started hiding USB exported playlists in RB5 Macbook (I've had this working for months)

I have a DDJ-1000 and a workflow for backups and Live Streaming.

My partner's older Macbook Air on Rekordbox 5.6.0 gets used for DJing sometimes as backup, or to free up better hardware for Live Streaming, leaving my faster Macbook Pro on RB6 containing my entire music library.

On Saturday and Sunday I had a paid gig and on the first day Rekordbox 6 failed me on my high spec Macbook Pro. This got fixed the next day by turning off Turbo Boost using external software and reducing waveform drawing speed to medium (FYI the sound was popping out of all the speakers, graphics jittery, and RB6 ground to a painful halt). Thats not what this post is about as I am testing the fix for now.

However, at the time I had no choice - Export today's playlists from RB6 to use the on the other RB5 Mac. And it worked all afternoon for 4 hours like I had done on several live streams in recent months.

The next day, I bought and added new tracks to the same playlists on my main RB6 Macbook Pro. But when I went to that RB5 Mac device with the RB6 prepared USB stick (not trusting RB6 until I test out the software) it showed the playlists again for 2 seconds and then RB5 hid them.

Thinking it was the USB, I Formatted the USB drive, Exported only 2 playlists I wanted quickly - same thing. 

I've tested this on a third Mac with Rekordbox 5.8.5 and Rekordbox 5.6.0 and it still wont' read the USB.

Does anyone know how to get RB5 to read the RB6 USB exported sticks again as I had them working for months like that. I've recently updated the increments from 6.02 to 6.1 etc and wondering if one of those updates has stopped older Rekordbox reading it's exports? Or is there a fix you guys can help me with?

Not having a USB-ready Pioneer model, I'm really stuck if I can't use backup machines when things go wrong on Rekordbox 6.

Thanks