For anybody with the same problema running an ASUS S551LB, I found the fix. You need to download and install the IRST driver from the ASUS page. It is installed by running PNPINST64: Driver
And then download and install ASUS' own version of ExpressCache: ExpressCache
Then follow the instructions in this link
>1.) Open an elevated command prompt and type diskpart.
2.) type "list disk" (note the drive number of the SSD).
3.) type "select disk n" (n= the drive number found above)
4.) type "clean"
5.) type "exit"
6.) Now that you are back to the normal command prompt, lets navigate to the ExpressCache directory. Do this by typing "cd C:\Program Files\Condusiv Technologies\ExpressCache\"
7.) type "ECCmd -partition" (this tells ExpressCache to use the whole SSD drive)
8.) You're DONE! to confirm that it is working, you can type "ECCmd -info" and ensure that it starts showing you some caching statistics.
Just make sure you reboot between installing IRST, ExpressCache and between *ECCmd -partition* and *ECCmd -info*.