ATA is also known as IDE and PATA - these all refer to the older protocol used by hard drives in systems up to a few years ago.
AHCI is the protocol by which the software controls a SATA drive - this is the newer protocol for drives used with systems built within the last few years.
With a modern system, chances are that all of your drives will be SATA drives and the software drivers that are loaded into your OS will use AHCI to communicate with them.
Older operating systems like Windows XP don't include AHCI drivers, so by default they can't use SATA drives. There are two ways around this:
1) You can install an AHCI driver by loading a floppy or CD during the Windows XP installation process.
2) On most systems the BIOS can be set to "IDE Emulation" mode. In this mode the BIOS makes a SATA drive "look" like an IDE drive, so that an OS without AHCI drivers can still use the drive.
If you're using a modern operating system (Vista, Windows 7, etc.) you should use AHCI and NOT "IDE Emulation" mode. AHCI includes some advanced features that improve performance and functionality.
WARNING - if you installed the OS with the BIOS set to "IDE Emulation" mode, then switching the BIOS to AHCI will render the system unbootable because the IDE disk drivers that it will have been using don't know how to talk AHCI. There's a fairly simple fix for this, let us know if you need it.
I would like to know, how to fix the problem with an OS installed with the BIOS set to "IDE Emulation" mode and then switching to AHCI. Because i think i might have that problem now. At least i can boot my computer.
So say for example I have a Dell XPS 630i (a little older 2008) and I "upgraded" to a Samsung 840EVO 500GB SSD and am unable to change BIOS to AHCI. It says SMART drive in my BIOS and wont let me change it to anything.. Am I SOL?? Samsung Magician throws an error stating "Magician cannot communicate with the Samsung SSD" it can however tell me the used space etc of the drive just no health specifics and system info. My PC seems to be faster but I definitely think it can be better.
Specs of PC:
Dell XPS 630i
Intel Quad Q6600 2.4 GHz
Kingston HyperX 8GB (4 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Quad Kit
Samsung SSD 840 EVO 500GB
2 More 500 GB Drives
Crappy Nvidia Galaxy Geforce GT 610 2GB DDR3 PCI Graphics card
Microsoft Windows 7 Home Premium SP1 64-bit