Hi Paul, sorry for delay.
I've made other researches to get deeper into both sata and esata operating mode and these are my results:
- sata/esata electrical signals are slightly different: Tx sata 400-600mV; Tx esata 500-600mV; Rx sata 325-600mV; Rx esata 240-600mV;
- maybe it's my mobo's fault because of the following taken from https://sata-io.org/sites/default/fi..._WP_11-09.pdf:
"Since many of the existing disk drives and Serial ATA chipsets were originally designed with only the internal signal levels in mind, they may not be able to meet the more rigorous requirements of the external interconnect levels."

In other words my theory is: for sure my fantec enclosure affords an esata signal from its esata ports; fantec can't "understand" there's an esata-sata cable connected so the esata signal is carried to the mobo's sata; my mobo's chipset (maybe fw?) doesn't recognize the slightly different signal so it's as the fantec is not connected.
What do you think of that? If it's correct do you think there could be a way to set the bios properly? I looked into the bios but I found nothing about that.
Anyway I guess it's not a good way to sold something to cable's wires to try to modify the transmitted voltages, is it?