If you have made an incorrect entry in the /etc/vfstab file, for example, when mirroring root (/), the system will appear at first to be booting properly then fail. To remedy this situation, you need to edit the /etc/vfstab file while in single-user mode.
The high-level steps to recover from improper /etc/vfstab file entries are as follows:
Booting the system to single-user mode
Running the fsck command on the mirror volume
Remounting file system read-write
Optional: running the metaroot command for a root (/) mirror
Verifying that the /etc/vfstab file correctly references the volume for the file system entry