OK, i got 1 for Christmas and I will just say I hate the I-tune software, it could be much less complex then what it is, just my opinion.
If you mean that you wish to erase music from your harddrive but have it on your I-pod that will work until you sync your ipod again and then I am pretty sure that music could be deleted. I am not positive, but I am pretty sure your I-pod will sync only songs you have on your list and if the file is gone from your harddrive then when you sinc it again these files will not be found and probably deleted from the I-pod. Just what I think I am still new to the technology and am still learning.
Good luck cause we may need it, hehe. 
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DX'ed with HFA as a child. However this was in 1987 and I am certain had I been DX'ed a few years later I would have been DX'ed with AS instead.