A while back I bought a used 20GB iPod off eBay and loaded Rockbox on it. Everything worked great for about 7 months but then it started powering up with the exclamation point folder icon. I could hear the hard drive trying to spin up but then it immediately shut down (clicking.) I figured the HD must be toast so I went back on eBay and bought a 60GB 4G color iPod (never used). I then put the broken one up for sale on craigslist and had a guy e-mail me. He said to give it a good thwack because the HD cable often comes loose on these iPods. Since I already had opened the case, I went ahead and checked the HD cable. I couldn't tell if it was loose, but I pushed down on it anyway. All of the sudden the iPod booted and Rockbox loaded up! Apparently the cable was loose because now it looks like the thing is working again (crosses fingers).

So, if you've got a 4th gen iPod that is displaying the folder with an exclamation point icon, you might try cracking it open and pushing on the HD ribbon cable.