Steve, unfortunately, I do not have the repair manual for the S-950, but after looking at mine, I am reasonably sure that the repair would not be all that expensive or complex. The USB socket is probably soldered on a PC board, and the board is likely anchored to the chasis by a couple small screws.
This said, if you have no electro-mechanical/electronic repair experience, I do not recommend undertaking this task yourself. Instead, take the keyboard to an authorized Yamaha Repair Center and have them do the job for you - just to be on the safe side.
Now, I rarely remove the USB drive from my S-950, and for the most part, unless I need to transfer some data from my PC into the drive, the drive remains in the S-950 just as if it were a miniature hard drive in the PC. I have an 8-gig drive that only protrudes about 1/4-inch out from the plug itself, therefore it is never exposed to breakage from someone inadvertently touching the USB drive itself. No extensions needed, no problems with access, very inexpensive, less than $15 US.
Good luck,
Gary