On Fri, 9 Sep 2011, Ted Ts'o wrote:

Rocko, Alan, could you try this patch and see what happens.  It may be
that we'll crash somewhere else; the problem is that Linux that the
low-level generic hd routines doesn't have a formal way of informing
the VFS and layers below that the disk has disappeared.  It just yanks
it out from under the file system, and we've been manually patching
around kernel crashes....

I confirm that this patch fixes the issue on with my test script.  The 
unmounts occurred with no apparent problems.

However you probably should make the same change to the ext3 driver, 
because it has exactly the same issue and some people may still be 
using it.

Alan Stern