Remember to attach images to thread:
http://www.digitalfaq.com/forum/news...ly-upload.html
Those "image linking" services are long-term unreliable.
Layer break issues are the most common kind of issues with dual-layer (double-layer) media.
Burning too fast is a common reason that even good discs will sometimes turn into coasters or semi-coasters.
So that begs the question... What speed were these burned at?
Generally speaking, it's best to burn at 4x, and no more than 6x. Being over an external drive, I'd suggest no more than 4x.
It would also be important to watch the buffers when your discs are burning. Were there any buffer underrun errors, where the drive had to halt and allow buffer refill before resuming the burn? That could cause several errors, including a bad layer break. (I don't think that's what happened here, but it's possible, hence why I'm mentioning it.)
If the drive is only failing on TFT burns, it could be a problem with the I/O (bandwidth, data throughput) at the higher speeds over the connection. Is this USB2? If so, remember that USB is a "burst" rated transfer, not a sustained connection. Therefore you'll have dips in the read speed.