Difference between ATA and AHCI?

AHCI is a controller interface while ATA is a standard for connecting storage devices. Also AHCI is hot swappable, meaning you can unplug a drive when it is up and running. You would need to reinstall the OS with AHCI to work and that would require an installation CD that has that default install configuration on it (The Drivers are completely different). You most likely do have AHCI enabled as I have seen this occur many times. AHCI is ideal for RAID and as I said hot swappable drives. I you are going to use your system like a server and it needs to be on 24x7 then AHCI is the way to go other wise ATA is fine.