Comment 14 for bug 26058

From http://technet.microsoft.com/en-us/library/Bb726994.aspx:
"You can't use dynamic disks with previous versions of Windows, but you can use dynamic disks with other operating systems, such as Unix. To do this, you need to create a separate volume for the non-Windows operating system."

See also http://support.microsoft.com/kb/309044

Is anyone _not_ using "dynamic disks" having this problem?

Please try the workarounds I linked to in my previous post. If one OS keeps rewriting the MBR, there's really not much another OS can do against it.