From LQWiki

-M does not exist as an option for me:

resize2fs -M /dev/sdz
resize2fs 1.40.2 (12-Jul-2007)
resize2fs: invalid option -- M
Usage: resize2fs [-d debug_flags] [-f] [-F] [-p] device [new_size]

It does for me on fedora 10:

resize2fs 1.41.4 (27-Jan-2009)
Usage: resize2fs [-d debug_flags] [-f] [-F] [-M] [-P] [-p] device [new_size]

-M stands for "shrink to minimum size" - Bemoeial 10:52, March 31, 2009 (UTC)