such information you give with your post its amazing.
What type of iPod, what generation iPod, what type of computer, what OS, what port are you using, what type of port are you using, have you tried resetting it, have you tried updating it.
If you've not enabled disk use, it will only appear on your Desktop when it's syncing, and unless you've made any (major) changes that shouldn't take much time the second time you connect your iPod.