The terminology is such that you [STORE] a Voice to the MOXF's internal User Bank.
You SAVE a File to a USB drive.

When you create a Voice, press the [STORE] button and direct to one of the 3 normal User Bank location (or if it is a Drum Kit, to the User Drum Bank)

For long term storage you can backup your User data to a USB drive connected to the MOXF.
Press [FILE]
Select SAVE
Set the File TYPE = ALL (.X6A which will backup all User data) or ALL VOICE (.X6V which will backup all User Voices).

You can Load a single Voice from either one of these files by setting the File Load TYPE = "Voice"

Thanks! how would I extract a voice I made from a USB though.

You can Load a single Voice from either one of these files by setting the File Load TYPE = "Voice"
You can view the User Bank folders, each contains a list of the individual Voices in that particular Bank
You highlight the name of the Voice in the right column (list), and direct it to a specific location in your instrument's current Banks shown on the left of the screen. You can direct it to the location you desire.

From the factory all 384 Normal and 32 User Drum Kits are "placeholders" and can be overwritten. Each appears elsewhere among the Preset banks.