ha! ha! Typical. As soon as you put a plea for help in a forum, you find the answer! Because I did an OS upgrade, Privacy&Security settings were changed. So I had to go into In System Preferences -> Security & Privacy -> Privacy tab ......Scroll down & select Full Disk Access, ...click on padlock icon to allow changes.....tick ON Allow Rekordbox...
I found the answer here: https://forums.pioneerdj.com/hc/en-us/articles/360043822571-Can-t-read-a-USB-device
Woohoo!! : )