Transfer Help?

I've done some upgrading on my MacBook and I need some help getting items from my external hard drive back onto my computer.
My MacBook is 4 years old and it was time to install a new hard drive and add memory. Before I changed the hard drive, I put everything I wanted to save on my external.
Now that I've finished with the hardware install and added the Snow Leopard software, I can't seem to get the data on my external to transfer back onto the computer, particularly iTunes.
I've tried to open iTunes and then drag and drop the iTunes folder from the external hard drive into the iTunes on the computer. It shows that it's transfering and then after about 20 minutes, I get a message saying that the "hard drive cannot read or copy", or something like that.
Any Ideas?
Thanks,
Chris

what filesystem is the external drive?

Oh geez, I don't think I know that. It's a Western Digital. Would it tell me somewhere on the external hard drive?
It worked fine before I did the upgrade so it must be Apple compatible?

OK, the message I'm getting says "Attempting to copy to the disk 'Macintosh HD' failed. This disk could not be read from or written to."

Sounds like a permissions problem. Boot your MacBook from the Snow Leopard DVD, enter Disk Utility from the top menu Utilities and repair permissions. Let us know if that works.