Itunes Not Always Playing Songs



I recently transferred all my songs from an old G3 to my new G5 by saving to an external HD and then copying across. I tried the firewire transfer, but it didn't work.

Now, lots of songs get an exclamation mark against them, and 'can't locate file message'. I just want to have all my tunes on my G5 and have them backed up on my external HD and be able to refresh the back up regularly. Is there something I can do, like, start from scratch?
I ran into this issue with a G4 to Mac Mini Itunes transfer. The easiest way (I found) to get Itunes to see all your music (rather than directing song by song to the appropriate folder) was to copy all the music to the new drive... then delete the Itunes preferences from the new machine. Relaunch Itunes so it thinks it's the first time it's being launched and then check the "Please find all MP3s on my Hard disk" (or something similar to that) Itunes will search the new HD and catalog all the music for you, so you won't end up with broken links. There could be an easier way to do this, but it did work for me.
try this way :

in iTunes preferences click the "advanced" tab
make sure that in" iTune music folder location" you have the right location (folder where songs are located in your G5)
if not change it.

FYI try to use the search feature of this forum I'm convinced this question has been asked already. Cheers