honestly, probably nothing...from personal experience no USB device has ever been damaged by this...still the manufacturers advise against it, so why tempt fate?

USB devices are stable; they aren't like RAM which is highly volatile. As such, you can, almost certainly, remove a USB device, any, for that matter, any time without worrying about damage to the device.

You know, it is luck of the draw because I have done this often and had nothing happen to my Ipod. last week, it fell out of my pocket at work and hit the floor. It still worked, no problem, kept on truckin. Then suddenly out of nowhere, it started to click, like I could hear the harddrive whirring in there. This morning, I unplugged my Ipod 80gb from my MacbookPro with Itunes open and did not hit the little eject Icon as I was in a hurry. In the truck on the way to work, I went to listen to Coast to Coast am podcast, and there was nothing, no playlists, no songs, no Podcasts, no data..nothing to be found on my Ipod. I thought I had lost all data on the Ipod. I plugged it back in, after work today, and everything I had saved on it when used as an external harddrive, was still there, strangley, just not the songs, playlists, etc. I copied it all to a new folder on the MB pro desktop, and will re format the ipod to original settings, then copy it back on and see what happens. If that doesn't do it, I have plan B, which is a good thing because I backed up my Ipod using an Itunes backup program I bought, onto the Macbook . Unfortunately, I reformatted the Harddrive since then (doesn't that always happen at the worst times?), but I should be able to get it back, using Mac's Timemachine program in OS X. If I do, I am a lucky man and a testament to how important it is to back up your stuff, regularly! Ideally, if your work is important, burn a CD or DVD of your important files, label the disk and put it somewhere safe, like a CD booklet made especially for backups, and keep it in a place where you can remember where you put it! (I am notoriously guilty of not remembering where I put my backup CD's) You will thank yourself for doing this because if you rely soley on a hard drive for backup, someday, it will fail, and do so when you have your whole life on the drive, 5 years down the road.