2500a: Unreadable Data DVD

Hi, I am trying to burn a data DVD containing mp3 and MHT files (html files). I use 2500a in a bytecc external enclosure (dual usb 2.0 and firewire).
I use both memorex DVD+RW (4x) and Imation DVD-R (x2.5), after burning these are partially unreadable and fails the verify data test on Nero 6.6.0.1

I tried with Recordnow! Max 4.5 with same results.

This happened with both FW 1.6 (original) and FW 1.7b4.

How can I resolve this? I will apreciate any help, thank you!