Yes you can use external Harddrive, but first you must format the drive as FAT32. You must download AOMEI Partition Assistant the free edition can be used for that. you can download it here : http://www.disk-partition.com/
It could be a quickformat and choose the standard clusters bytes.
Never pull out usb cable when database or track is loaded, use safe usb remove stop button on cdj's ans pc's Always or your database can be destroyed or corrupted
But fact is external Harddrives would be slow, i recommend a small SSD drive. i use a 256g & 512g from transcend. You can format them also as FAT32 and transfer speed from recordbox to SSD drive is very fast (with usb 3.0).
It's also very fast to load your database & tracks + you don't need external power adapter (usb power is ok).
Once the SSD is Working it would work on every CDJ unit. I'm using the old cdj-2000 & XDJ-1000 units Greetz Dj Smoke