Fsck

fsck acronymic for file system check. Typically run with the -y switch to check the filesystem in question and fix any errors it finds. FreeBSD, like most unixes, doesn't immediately write to disk. It first stores any writes to temporary memory -- RAM or swap -- and then later writes the changes to disk. This is handy because it's fast. The tradeoff is that if you have a pending write and your system shuts down before it gets done -- say a power outage happens -- your system may suddenly have a corrupted filesystem or partition.