First off. It could also mean that the drive you're writing to has some bad blocks or issues.
Being that it is formated ext3, I would dismount it and force run fsck on it. Then, remount it and try again.
Also, try to copy the file to the blackhole and see if you succeed copying the entire thing. If it fails, you obviously have a problem with the source drive.
try: cp 70GBfile /dev/null
Let me know how you make out!