Ditto on the formatting - it'll be a lot more reliable and faster if you format for OSX.
I've also found that file transfers will fail if the internal hard drive (the one the OS is running from) is almost full - it'll fail transferring to the external.
Weird, but it's happened on several occasions prior my upgrade to a 1.5TB internal.
Just in case - it'd help if you mention the make/model of the drive, computer and what interface you're using. (Firewire 400 / 800 / USB) ect...