do u have a window$ Os running,i'll say try fixing ur window$ partn using chkdsk utility avail in winxp.
for ubuntu to not to show that message,u may edit /etc/fstab and
Code:
<file system> <mount point> <type> <options> <dump> <pass>
/dev/sda5 /media/sda5 vfat defaults,utf8,umask=007,gid=46 0 1
have the last lines as both 0 and 0.