iTunes won't let me move files to new hard drive

Hey, all...I'm having a really major problem with my computer. I'm trying to move my iTunes library to a new hard drive. Currently, it's in a 1-terabyte Western Digital USB drive powered through the USB, and I'm trying to move it to a Seagate 1-terabyte drive. I currently have 535 gigabytes of media in my library and I have a MacBook with a 250-GB hard drive that I don't store any music on currently.

I move the library file over to the new USB hard drive, and I consolidate my library as it asks me to do. Then, when I go down to "Organize library", it prepares to copy the files to the new disk, then gives me the big red stop sign and tells me "The disk cannot be read from or written to." and that's that. The files work fine when I try to play them, so they're there, and presumably, not irretrievably damaged. They're just impossible to move.