"unknown device" with Factory Cable

Hi!
I'm desperate.
At last I could find a "Factory Cable" to flash my XT910 with "Battery Low" message....Now, with this cable a beautiful "Battery OK" appears when I use the Ap Fastboot Mode to flash it with RSD Lite, but....my computer doesn't detect it!!!!! And if my coputer doesn't detect my phone, RDSLite also, and I could't flash it
What should I do???
I've tried in 3 different computers (with W7 and XP, all with last Motorola Drivers installed)....
I plug the factory cable into the phone, plug it with the computer, and press "Vol- Vol+ Power" ...then "Vol- Vol+" and I enter to Ap Fastboot mode....
"Battery OK" appears...
Windows tries to install the phone but....."Unknown device"
Please, what am I doing wrong???

Thanks!!!!