playing Music from Hdd in rkrd ,djm750 sound card connectivity

hi pioneer staff ,plz i want to know if it is applicable to connect ext hdd to rekordbox dj and play my imported library in the hdd as in cdjs.

in addition i have djm 750 in my lounge bar every time i connect the the djm sound card by usb cable i check my audio in the settings for rkbd i don't see the djm sound card only the built in sound card i installed all the related drivers that comes with rekordbox dj for djm750, but till now cannot figure it out i also turned the channels in the mixer into usb mode for each.so really don't know how it works . i have macbook ,el captain OS X, and now am upgrading sierra OS X ,so am i missing something. thx