Hey @AlexiHusky, Dropbox is certainly not required. It’s just an extra source you can use to play music from. You can play tracks offline from USB, SD, and internal SATA drive bay. Dropbox is treated as its own Device in Engine DJ.
It’s going to be a lot of typing to go through all the possibilities to get music to a source, but everything is well documented in the user guides. I would recommend the to study both the user guide of Engine DJ (desktop) and the SC6000, both linked below.
It’s also possible to disable Dropbox completely. In the desktop software you can do this in the Preferences → Library → Integrations and turn Dropbox off.
On the hardware press the Shortcuts button or swipe down from the toolbar to open the Control Center → Settings → Services → Disable Dropbox in Cloud section.
Hope this get you started for now, but if you have more questions feel free to ask