Few, if any USB devices are recognized by the Prime 2.
Over the past six months I have found only one mini-USB of very limited storage capacity that will work with the Prime 2, but I have gone through various USBs (brand new SanDisks, EasyStore USBs) all formatted to MS-DOS FAT 32 and none of them are recognized. I have had parties at my house which were cancelled (we ended up just playing a bluetooth speaker rather than playing our sets) since all guest DJs USBs could not work with the Denon DJ Prime 2, but are readily recognizable on Mac/PC and other controllers.
At this point, the Prime 2 has just been collecting dust while I research a fix in my rare spare time between being a single dad and work.
I have been trying to troubleshoot for the past six months before throwing in the towel. After the 5th brand new USB not being recognized I am contacting you (Denon).