@GFOXX
Please attach or give us a link to the latest original BIOS for your Zenbook device and don’t forget to give us the exact model name (there are obviously different variants of the “Zenbook Pro UX501J” available).
It is very unlikely, that you will get a much better performance with your old Zenbook device by replacing the on-board M.2 SATA II SSD by another (more modern) SATA III one. The maximunm AHCI data transfer speed is limited by the mainboard’s chipset.
@Fernando What makes you think that he has a SATA II SSD (SATA III came in 2009)? I doubt there were built any M2 SATA 2 SSDs?
@GFOXX What model of storage device do you use at the moment in your Asus Zenbook? Please attach a picture!
You are right and I was wrong. Indeed: A SATA III M.2 SSD should be supported by the Asus Zenbook Pro UX501J series (provided, that the BIOS supports a SATA M.2 SSD at all) , but the speed will be limited by the on-board PCIe 2.0 interface. When I wrote my reply, I obviously mixed the SATA with PCI interface. I am sorry about that.