Hi Toril,
The User drive on both the PSR-S970 and the S975 is the same size, and is approximately 13 MB.
Although this is more than any previous PSR-S, or even Tyros models, it is still a relatively small amount of memory (your Tyros5 has only 6.7 MB although of course it also has a huge hard drive available)!
Although you say that you don't have any expansion voices installed in your S975 at the moment, if you do ever load any, although the wave samples used by those voices are loaded into the Expansion memory, a small amount of space is also required on the User drive for the voice parameters, so you may need to take this into account at some point.
You can connect the keyboard to a computer via USB and use Yamaha's 'MusicSoft Downloader' program to check everything on the User drive more easily than using the various filtered selection screens of the keyboard OS.
You will then see exactly what is using up the memory, whatever the file type. There is presumably more than you think!
You should really think about setting up a USB drive as your main storage drive. A small footprint drive of the type suggested by Joe H could be left in the keyboard all the time with little risk of damage to the keyboard or drive. Although I personally would not trust drives bought from ebay as being genuine, drives from reliable sources are so inexpensive these days, so you could even get two of them and make a copy of the working drive to carry with you on gigs as extra security if you are really worried about forgetting the working drive.
Regards
Ian