Upgrading Hardware & Windows 7 Can't See the Disks


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I just ran into this after a motherboard+CPU+RAM upgrade.

I'd already sorted things out in Linux, so had the BIOS set to use 'AHCI' (SATA standard protocol) for the hard disks. However it turns out that when Windows 7 installs it figures out which drivers it needs and the explicitly will only use those... even if it can't see any disks at all at boot time.

On trying a safe boot with command prompt it would scroll up the list of drivers it was initialising, get to CLASSPNP.SYS, pause a few seconds, and then reboot the machine.

So, what you have to do is:
  1. Go back into the BIOS and set the disk interfaces to 'IDE'. This way Windows should see them using a bog-standard old EIDE driver, even if it's crap and slow.
  2. Boot into Windows (this worked for me, you're stuffed if it doesn't), and install the drivers supplied with the motherboard (or add-in card if that's what you're using).
  3. Probably best to reboot at this point to ensure the drivers are settled in (I was forced to anyway by the installer).
  4. Now you'll need to fire up regedit on "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\msahci\start" and modify the value (it was '3' on mine) to '0' (that's zero). This will cause Windows to actually try all installed drivers if needs be on the next boot.
  5. So reboot, by way of the BIOS menu to set the disk interfaces back to AHCI.
  6. Cross-fingers and hope Windows now works, it did for me.
I should mention that I found the registry key hack fix elsewhere on the internets.
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