Run chkdsk with the /f switch.
From windows, click Start, Run, type cmd in the box and click OK
At the command prompt, type the following replacing X with the drive letter of your Sansa:
chkdsk X: /f
You will see something like this if all is well:
C:\>chkdsk g: /f
The type of the file system is FAT32.
Volume Serial Number is A84D-95CE
Windows is verifying files and folders...
File and folder verification is complete.
Windows has checked the file system and found no problems.
1,984,987,136 bytes total disk space.
1,376,256 bytes in 5 hidden files.
454,656 bytes in 95 folders.
1,711,091,712 bytes in 1,310 files.
272,056,320 bytes available on disk.
4,096 bytes in each allocation unit.
484,616 total allocation units on disk.
66,420 allocation units available on disk.code]