SOLVED: thanks for all advice. The issue was actually to do with the way the USB was formatted. More info here: DJ Tips & Tricks: How To Format A USB Drive For CDJ Use.
I used this tool to reformat: https://fat32-format.en.softonic.com/
SUMMARY:
"modern Windows computers use the NTFS or exFAT system, and Macs use the HFS+ (MacOS Extended Journaled) system. Macs can read NTFS thumb drives, but can’t write to them. PCs can’t read or write to HFS+ at all.
However, there is one file system that both Macs and PCs can read and write to: FAT32. Pioneer DJ’s CDJ media players are also able to read music files from drives formatted with FAT32 (FAT 16 and HFS+ are also included)."
Hope this helps someone.