Waaov, lots of valuable comments. Thanks for your interest.
Here are my comments:
The data on the discs are retrievable for now. I completely copied the disk content back to the computer.
I plan to update the firmware, but because my computer is a relatively slow one I preferred to burn at 2.4x.
But I do not think that the bad result is because of the firmware or burning speed.
I burned the similar media of other brands and they gave much better results.
For example Philips, Maxell and TDK (1-4x DVD+RW, sold in jewel cases).
(I can post some scans of them also)
But of course they were more expensive that Verbatim (mostly becuse they are sold in jewel case boxes)
That is why I think that companies sell their second grade productions as spindles.
Finally, erasing the disk before writing anything seems as a good idea.
I shall try it.
Thanks and regards