I think this is an issue with the Mac OX 10.4.x Finder and large files on certain models of external hard drive.
Whenever I run into this problem, I've always been able to work around it by opening the file (from the iRiver) using Sound Studio (importing can take a l-o-n-ng time, and appear to freeze up, but unlike the Finder Sound Studio won't give up), and then once the file has been read in fully choosing Save As... to put the imported file on my hard drive. But this didn't solve a similar problem for someone else.
Be sure your iRiver is well charged or on AC power, as this takes quite a bit of juice.