Success at last!!!
Used an external HUB and a new OTG cable [credits to Kholk].
Didn't worked with the external power from the HUB (actually didn't recharge the Razr).
Worked using Y wire.
Could only access pendrives when connected to the HUB.
Very strange as I supposed it should also work just connecting directly any memory stick to the OTG cable (powered with the Y cable)! Instead an USB HUB is required!
Checked back and actually this configuration works also with the samsung cable (this confirms to me its a real OTG cable, I was so desperate that came to thinking it was just plain plug converter).
So maybe I made 3 working OTG cables, but was checking the wrong configuration!
To my great surprise the Razr dretects when the device is connected (just wait for a bunch of seconds) and lets you also explore it and unmount it even without using webtob (just select the option from the USB Icon that will appear in the upper bar).
The good news (not to me as I wasted a lot of time soldering) is that this setup can be obtained with standard cables:
-Any real OTG micro USB male to USB female
-Any USB male to mini USB male Y cable with extra USB male just for power (standard for some external HDD)
-An external USB Hub with female mini USB in.
Before updating my first post will do some more experiments and let you know about the findings.
Motorola Razr XT910
HTC Blue Angel