(After further input, I had to rewrite my answer.)
See if this Microsoft Support article applies to your case:
Windows Server 2008-based or Windows Vista Service Pack 1-based systems do not generate a dump file if the system partition is a mirrored volume
The article says:
This problem occurs because the
Crashdmp.sys driver checks whether the
volume of the system partition is
supported when it is loaded. However,
the Crashdmp.sys driver performs an
incorrect check when the system
partition is a mirrored volume.
Therefore, the Crashdmp.sys driver
identifies the mirrored volume as an
A supported hotfix is available from
Microsoft. However, this hotfix is
intended to correct only the problem
that is described in this article.
Apply this hotfix only to systems that
are experiencing the problem described
in this article. This hotfix might
receive additional testing. Therefore,
if you are not severely affected by
this problem, we recommend that you
wait for the next software update that
contains this hotfix.
This hotfix is available for download from the above article.
The problem with this article is that it talks about disk mirroring. Do you have that?
Please note that Windows Server 2008 is mentioned specifically, which has a common code-base with Windows 7. You may contact Microsoft Support to find if it applies to you. If you decide to apply it, I would suggest backups and create a system restore point before, just in case.