USB Drive not showing up correctly in 8.1 (works in 7.3)

[Steve Randall]

On Sun, 16 Jan 2011 11:00:01 -0700 (MST)
Warren Block <wblock at wonkity.com> wrote:

> On Sat, 15 Jan 2011, perryh at pluto.rain.com wrote:
> 
> > Jerahmy Pocott <quakenet1 at optusnet.com.au> wrote:
> >
> >> I have a USB Drive that was working fine under 7.3, but since
> >> updating to 8.1 no longer has the correct /dev entries. Under
> >> 7.3 it was da0s1, in 8.1 there is now only da0 and da0a, which
> >> shouldn't exist...
> 
> da0s1 is MBR, da0a is "dangerously dedicated".  I would not expect 
> differences between 7 and 8 USB to show that on an existing drive.  If 
> the drive was reworked during the 7-to-8 upgrade--maybe with "bsdlabel 
> auto"--that would be a more likely explanation.

The actual explanation is the new geom partitioners introduced in 8.0.

The disk has an MBR in block 0 and a forgotten BSD label (dangerously
dedicated) in block 1. 8.x sensibly ignores one of them; unfortunately
it ignores the MBR, not the unwanted BSD label.

The solution is:

# dd if=/dev/zero of=/dev/da0 seek=1 count=1