Quoting Andrew Raibeck <storman AT US.IBM DOT COM>:

TSM does not "lock" files per se; however, it does open the files to read
them during backup. It could be that SQL server requires exclusive access
to the file, so if TSM is currently backing it up, SQL server cannot
start.

Hi Andy,
thanks a lot for this answer. Useful and complete as usual.
Unfortunately, the Windows team is late on hotfixes about VSS/COM+ here.

David