iTunes and iPod 20GB - exclamation mark and

I have just bought a 20gb ipod and I'm in the process of transferring
my existing Mp3 collection. Can anyone tell me why I receive an error
message such as "Some of the songs in the iTunes library, including the
song <song name>, were not copied to the iPod <ipod name> because they
could not be found." ?? The only thing I can figure out is that the
files that struggle have an exclamation mark next to them on the left
hand side. I'm finding it very strange and I'm not sure what to do.
I'm not doing anything complicated. Here's a summary of what I'm
doing:

FYI my MP3 collection is all organised by Artist, Album name and each
album has an m3u. So all I do is:

1. in iTunes select File|Import then browse to album's m3u file.
2. the album is then imported in to iTunes.
3. I update ipod, then error message appears. If I go to a music file
that is mentioned in the error message I see the exclamation mark. I
right click on mouse button and select "get Info" and the error message
"The song <song name> could not be used because the original file could
not be found. Would you like to locate it?"
4. I browse to the file and the exclamation mark dissapears and that
file then updates on the ipod, however the next file in the album
suffers from the same problem and so on.


Any ideas why that message is appearing, when I have only just imported
it and I haven't moved any files?

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