Sansa m250 2GB can not see any files or play

I had the same problem. I couldn't do "natsw"'s fixit. But I did something that I think worked so far. I have a m250 which worked for a bit, then when I added new tunes from a different computer, I got the Needs 4 MB free space when in fact I had way more free space than that. I simply connected the thing to my non-Mac computer, right-clicked on the m250 icon and chose "format." I reformatted everything which basically deleted everything on the m250. I then added a new folder called "audio" and added new songs to that folder. So far, so good.

I had a similar problem.
This is how I fixed Sansa m250 2GB
I have Windows XP, so if you have a different version of Windows you might have to do something else.
Connect the player to your computer.
From My Computer open the drive that says SANSA M250
If there is no folder called "audio" make a folder named that. (this is where your future mp3 files will be stored).
My audio folder was missing, so I had to do this.
Now go back to My Computer right click on the SANSA M250 drive scroll down the menu and open the folder that says "Open As Portable Media Device".
Now click on the icon that says "SANSA M250"
Now open the folder that says "FOUND.000".
If you are having the same problem that I did all your old mp3 files will show up with a file extension .CHK instead of mp3 and the names will be numbers. You can either erase all these & reload all your songs into the "audio" folder I mentioned previously or you can rename each file extension to a .MP3 instead of .CHK. (after you change them to .MP3 they will load on your mp3 player. But, remember if you do choose to rename them instead of erasing and reloading, when you want to permanently delete them you will have to access the "FOUND.000" folder again to do that.