OK, first and foremost, just plug a USB storage device into the back of the keyboard. Anything 3 GB or larger will suffice. The keyboard will immediately recognize the device.
Next, the only files you need to back up are third-party/user style files, registrations and user voices.
Press the Expansion/User button at the lower right corner of the style section of the S-950. When that screen opens, press the bottom of the number 3 button at the bottom of the display (Copy), then select All (bottom of #6), OK (bottom of #7, the tab to the USB1 drive and press Paste (bottom of #4) and the information will be copied to the USB drive.
Next, repeat the same process with the Expansion/User voice section on the right side of the keyboard.
To copy registrations, press both the + and - registration buttons at the same time. This will open the Registration folder. Again, using the above procedure, copy and paste ALL the registrations to the USB1 drive.
The Music Finder Directory is a totally different operation. To access the music finder directory folder you must press the Function button, located on the lower right side of the display, then select Utility and tab to System Reset. Next, press the I button next to the block that says Music Finder Files, this will open the Preset Music Finder Screen. Now, tab over to the USB1 drive and press Save (bottom of #6 button), provide it with a new name such as MFD 2017, and press OK.
All of the other things in the keyboard are stored within the keyboard's onboard memory and while they can be saved, they are onboard style and voices files, which you can not reinstall if the keyboard were to have a complete meltdown or system failure.
Hope this helps,
Gary