The self-destruct message is one of those never-supposed-to-appear
types. It means that the device appeared to unmount successfully, but
afterwards there were still busy inodes for that device.
It probably means there's a programming error in the msdos fs that's
leaving an elevated inode use count. Unfortunately, the inodes don't
really self-destruct, so there's a small probability the msdos inode
could get used with whatever you mount next on the same superblock. (Not
good.)
Is this problem repeatable for you? Someone familiar with msdos fs
should track it down, and it would help to have a test case.
Regards,
Bill