As this media wasn't known from the regular firmware (VS0M), I downloaded the CG5J and tweaked it with omnipatcher, replacing RICOHJPN-W01-00 with RICOHJPN-W21-01 (info by: NeroInfoTool) with 4x burning capability.
[Formating the media never complained and always said it is Ok.]
Now NeroExpress tell it burns @ 4x (was 2.4x before installing CG firmware); k3b, under Linux, also say that, but the real-time speed probe says it burns @ ~2.0x.
In both softwares everything seems to happen correctly BUT neither NeroExpress nor k3b can verify the burn: the disk is empty (and it seem effectively blank at sight).
Inserting the DVD in any DVD-ROM drive read it as empty.
If you have any clue it will be welcome (I've to backup a disk, and don't want to use regular DVDs)
Thanks in advance,