I reformatted a hard drive to ext4, planning to use it as a backup drive. After mounting the freshly-formatted drive, I discovered a single empty directory inside it: lost+found. What's the purpose of this mysterious directory?
lost+found is the directory in which fsck (filesystem check) will put files it restores from orphaned blocks. This can happen when something corrupts filesystem meta-blocks (also called i-nodes) in which the references of the blocks are stored which contain the data of a file.