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Date: Mon, 4 Jun 2012 19:26:32 +0200 (CEST)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: Kaya Saman <kayasaman@gmail.com>
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Cc: freebsd@dreamchaser.org, Thomas Mueller <mueller23@insightbb.com>,
	freebsd-questions@freebsd.org
Subject: Re: Strange case of vanishing disk
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>
> I just "offlined" the system and took a look at the BIOS. The hard disk 
> controller was set to "ATA NATIVE", I attempted changing to AHCI and the 
> system failed to boot thereafter.

do you have ahci and ada drivers compiled in?

>
> Booting into my rescue CD of FreeSBIE, with AHCI enabled only showed "ad4" 
> (the troubled disk) to be registering.

showing up some, but not all disks after rebooting is quite common to the 
problem i described.

POWERING OFF (by disconnecting electricity, not by power button), waiting 
a minute, and powering on "fixes" the problem for some time.