Trying to understand what this issue is. You are loading in Voice libraries (we assume you have DIMMs installed - you don't mention how much memory you have installed, nor do you tell us the size of the FILE you are trying to load) The Motif XS should be able to give you have information.

Then you say when you "go to save the files it gives me a 'sample memory is full' message"... The Motif XS only gives such a message when you are attempting to LOAD data from a FILE... never when you are trying to SAVE data to a FILE. So we are a bit confused.

Are you loading or saving. LOAD is the act of moving data from a storage device like a USB stick into the Motif XS.
SAVE is the act of moving data from the Motif XS to a USB stick storage device for long term storage.

I'm somewhat new to the Motif.

I am trying to LOAD sounds from the USB stick to the Motif. The file size 430MB. I am not sure how much DIMM space there is available (I purchased this Motif used from Guitar Center).

What do you advise?

You can check on the amount of Sample RAM
From the Owner's Manual...
...go to the Sampling display by pressing the [INTEGRATED SAMPLING] button, and press the [F6] Rec button then [SF6] INFO button. If the DIMMs have been installed properly, the appropriate available memory size is indicated in the display.

See page 295 of the Owner's Manual for details on the DIMMs.