Some of his largest contributions to BSD have been to the file system. He helped to design the original Berkeley Fast File System (FFS). In the late 1990s, he implemented soft updates, an alternative approach to maintaining disk integrity after a crash or power outage, in FFS, and a revised version of Unix File System (UFS) known as "UFS2".
The magic number used in the UFS2 super block structure reflects McKusick's birth date:
#define FS_UFS2_MAGIC 0x19540119 (as found in /usr/include/ufs/ffs/fs.h on FreeBSD systems). It is included as an easter egg.).