Sata Controller failure on iMac 21.5" mid 2010?

Hi,

I've got a problem on my iMac and I guess it's related to the sata controller.

1. My iMac started to boot really slowly and to be slower and slower.

I started to think it was hard drive related.

My harddrive 500 Gb and it is just 50% full. I used it for the system only. the data are on external hard drive.

2. I ran disk doctor to check the integrity of the hard drive, but there were nothing wrong.

3 .So I cleaned the cache. Nothing changed. later the iMac took ages to boot.

4. So I decided to plug with a Firewire cable my iMac to my mac mini and started it in target mode.

I've checked the harddrive. Everything was fine. I took advantage to make a back up of the same hard drive.

In the meantime, I've check the 16 Gb ram of the iMac, maybe one the ram is defective. The ram is fine.

5. I decided to reinstal the system. this computer was still on Snow leopard. The DVD was running fine then the install stalled at the early stage of the process.

At this moment I started to think there is something wrong with the sata controller.

6. So I used an external hard USB and install my backup on it and I booted up the iMac. The iMac is running on the external hard drive.

As far as I know the sata controller is a different chip than the USB controler. The firewire is working fine too. I have no thunderbolt device to test.

The drawback of the USB2 external harddrive, the transfert data is a bit slow compare to the internal hard drive. So the mac has lost a bit of reactivity.

I want to fix this iMac, but I'm wondering if that mean, I need to change the complete logic card?

Do you agree my problem is Sata controller related?

Do you think the only way to fix it is too change the logic card which might cost about $800 with no tax?

Do you think, an alternative solution is to use a thunderbolt or firewire harddrive for my system?

I thank you for all the suggestion.