I wouldn't call unchecking one preference a "total nightmare".

If you're not expecting that, and have mostly empty or invalid ID3 information, because you have habitually kept metadata in the filename and directory hierarchy, iTunes will destroy your library. Everything becomes Unknown Title 243 by Unknown Artist 14, and by the time you notice (because who's expecting an MP3 player to rearrange all the files on disk!?), the only way to fix this is by listening to each track and trying to figure out via Google what song it is.

This happened to me in 2003 when I switched from Linux to Mac. I still get a little worked up over that, even though nowadays I just let iTunes manage everything and throw away the original files after I rip something.

However, Mr. Figs, and this is speaking from personal experience, if you happen to store your files on an external USB drive, and you happen to launch itunes at a moment when the drive is disconnected from your laptop, Itunes will revert to its default folder--forcing me to think about things I shouldn't be polluting my brain with.

I actually find that handy for use with my laptop--unplug the HD, import my "core library" (about 30GB), then plug it in and import the rest (over 200GB at this point.) Thus, when I'm out and want to listen to something, I still have all my favorite songs; just not everything (which wouldn't fit on my laptop's HD, anyway.) Basically, it turns the internal HD into a not-too-smart cache of the external one. (On that note, if there's a program that provides this internal/external storage caching mechanism more intelligently and on a larger scale, I'dclove to hear about it.)

As in the cases people already described, and some others I won't go into, it is NOT as simple as unchecking a preference.

Also, try plugging your USB hard disk into a different USB port in a different order from your iPod. Now it comes up with a different drive letter and iTunes can't find any of your music.