Intel RST : acceleration is not available

Hello everybody,

Recently we purchased some HP 8300 elite computers. They were shipped with two disks, as this :

1x 20 GB SSD hard drive

1x 500 GB SATA hard drive

the SSD is used as a cache-disk and normally we should be able to accelerate the other disk (where System is installed). I installed Windows 7 Pro 64-bits on this computer, downloaded and installed the latest version of Intel Rapid Storage Technology software but I don't have the option to accelerate my disk.

Please see below :

1. in Windows Disk Manager (only one system partition) :

2. in Intel RST homepage, I can see my SSD-cache and system disk:

3. but in Performance tab I cannot accelerate my system disk, it is not present in the right-sidebar and the button "accelerate" is missing:

Any idea what could be the problem ?

Thank you in advance for your help.