Limited speed on SATA (with SSD)

I search lots of sites and posts but still can't find the solution. Pls allow me to repost a common old question.

I got an old iMac which is a late 2009 iMac (21.5 inch).
I added some ram (total 8GB now) and replace the optical bay with a SSD drive (256GB Samsung 840 EVO).
And then I form it as a fusion drive with the old hard disk (1TB 7200).

So far so good expect that the negotiated link speed is always 1.5 Gigabit.
I checked the web but I can't find a solution for this. Anyone did solve it in the past, please help to give me some insight to do so. Many thx in advance.

Vendor: NVidia
Product: MCP79 AHCI
Link Speed: 3 Gigabit
Negotiated Link Speed: 1.5 Gigabit

photosmike said:

Make sure you have the latest firmware

http://support.apple.com/kb/ht1237

My bad. Just noticed that there is not a firmware update for the late 2009 21.5" iMac.

Click to expand...

thx for your sharing. I accessed this page in the past. But late 2009 21.5 inch model is not on the list (but 27" is on the list). Still can upgrade?