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I have a laptop that already has an SSD and an HDD. I want to replace the HDD with another SSD. I have read that I need to change the BIOS settings to AHCI.

I want to ask this because my PC has an SSD from the factory with the OS already on it. Does this mean that AHCI is already chosen or should I change the setting for the second drive as well?

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I have read that I need to change BIOS settings to AHCI.

First of all check if it is not already on AHCI. Usually you have these choices:

  1. AHCI (The normal mode. You want this one)
  2. Ancient IDE compatability mode.
  3. On some systems only: Fake RAID.

Any laptop bought in the last decade should come with AHCI enabled. It allows extra functionality and is often slightly faster when you use a rotating HDD. It can be a lot faster if you use a SSD since it allows you to queue commands and SSDs perform a lot better when the queue depth increases.

should I change the setting for the second drive also?

The setting is typically set for all devices. I never ran into one where you could set on device to use AHCI and another drive to fall back to compatibility mode.

Not asked, but if the laptop is for some reason not-preset to AHCI mode then changing this might make your system unbootable. If you have a Linux or BSD OS it probably just works. If you have XP you might need to reinstall XP after changing the setting. If you have win7 you need to enable the msahci service, shut down, then change the BIOS setting. (In that order).

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