Some filesystems are mounted from remote machines.
Most UNIX systems today support some sort of remote file sharing . The most common of these is the Network File System, or NFS.
Under NFS, you can mount a directory from another machine as a filesystem. This means that the actual files reside on a disk elsewhere on the network — perhaps on Norm’s machine down the hall or on Eric’s machine on the other side of the country.
At our company, we have two main offices, one in California and one in Massachusetts. When users from California come here, they find all the files that they use every day, remotely mounted onto our machine. The files are still being read and written on the hard disk in California, but users can work on them here in Massachusetts — it just takes a little bit longer to open and write files and to list directories.