install/49128: Reinstallation fails with "Device busy" since wedges enabled

[gson%gson.org@localhost]

>Number:         49128
>Category:       install
>Synopsis:       Reinstallation fails with "Device busy" since wedges enabled
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 19 20:00:01 +0000 2014
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2014.08.18.17.16.42
>Organization:

>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

I tried to do a sysinst install of -current on a physical machine,
replacing an earlier version of -current that was installed on its
hard disk, and the install failed at the point where sysinst runs
newfs to create the root file system, with the error message

  newfs: /dev/rwd0a: open for read: Device busy

The dmesg from the INSTALL kernel contains:

  wd0 at atabus2 drive 0
  wd0: <IBM-DTTA-351010>
  wd0: 9671 MB, 19650 cyl, 16 head, 63 sec, 512 bytes/sect x 19807200 sectors
  dk0 at wd0: wd0a
  dk0: 15610896 blocks at 63, type: 4.2BSD
  dk1 at wd0: wd0b
  dk1: 4196241 blocks at 15610959, type: swap

The automated installs on the testbed are working, but I suspect that
is because they are done on initially blank virtual disks where the
INSTALL kernel will autodetect no wedges rather than on a disk with
a preexisting disklabel.

>How-To-Repeat:

Install -current using sysinst on a hard disk that already has NetBSD installed.

>Fix: