Hey Okazar, yes I've been able to successfully burn DVDs from ISOs located on an external hard disk. Seems to me your discs aren't getting finished properly. That's why the data looks like its there, but the disc remains unreadable. My guess is that your computer can't handle data running to and from the burner and the external hard disk at the same time.
Play it safe dude. Copy the ISO to your onboard hard disk, then burn from there. Sorry.
How fast a system are you running? If its an older system then it probably can't sustain enough throughput off the usb to keep the burning happy.
When you are burning do you ever notice the buffer running low?