My desktop machine is built on Gigabyte GA-Z87-UD3H and Gigabyte provides the latest driver for Intel Rapid Storage Technology (IRST), which I installed after installing the OS. Same goes for my Lenovo Thinkpad-T420. And for both machine, checking the controller device under the IDE ATA/ATAPI Controllers section in Device Manager I see the driver has been updated to the latest version.

• I set the SATA controller to AHCI from BIOS
• On the desktop machine I have one WD 2TB BLACK & one WD 3TB Green
• I don’t use RAID, & no chance of using in near future, but according to Intel IRST improves performance in single disk scenario too.

Now I have the following questions –

• What is the actual purpose of IRST (pre-OS install) driver that doesn’t get served with a post-OS driver that I installed? There must be some difference, otherwise there wouldn’t be a pre-OS version of the driver. Right?
• In the pre-OS procedure (loading the drivers at OS-installation time) after successfully completing the OS installation, do I need that post-OS driver? Because after installing from that one I got a quick launch icon that runs the IRST configuration application. Where do get that after installing the pre-OS driver?
• As it is “pre-OS”, when I load it at OS-installation time, does it updates anything at BIOS level or anywhere other than HDD? That’s because I’m going to dual boot Windows 7 with Windows 8.1, and after installing Windows 7 when I install Windows 8.1 & load the IRST driver for that, is there any chance of any “overwriting” or OS-incompatibility? In short, is there anything specific to follow while installing the second OS?