There are times when you may want to back up all files in a single directory, but you may also want to limit the selection of files to those on the same mounted filesystem as one of the files in the files parameter. You can do this by including the -m option on the command line when you create the archive. For example, this command line will back up only files in the root directory and exclude any files on mounted file systems:
bru -c -v -m /
The next example saves all files on both the root and the /usr filesystems, but none of the files from other filesystems, such as /usr2, is included:
bru -c -v -m / /usr
If you are not certain which file systems are currently mounted, you can use the df command to list this information. The output of df varies from one UNIX system to another, but will generally list one filesystem per output line.