ZFS: Adding a new mirror to an existing ZFS pool

  Mirrored vdevs are great for performance and it is quite straight-forward to add a mirrored vdev to an existing pool (presumably one with one or more similar vdevs already):  
zpool add [poolname] mirror [device01] [device02] [device03]
  If it’s a two-way mirror you will only have two devices in the above. An example for ZFS on Ubuntu with a pool named seleucus and two SSDs could look like:  
zpool add seleucus mirror ata-SAMSUNG_SSD_830_Series_S0XYNEAC705640 ata-M4-CT128M4SSD2_000000001221090B7BF9
  As always, it’s good practice to use the device name found in /dev/disk/by-id/ rather than the sda, sdb, sdc etc. names as the latter can change – the former do not.

Ubuntu: How to add or append a file to an existing tape backup

  Sometimes you may with to add files to an existing backup; if you issue a command like:  
tar -cvf /dev/st0 backupfiles
  …and the tape is not already set to the end of the previous archive you will over-write any data from the position on the tape. Use the “eom” command to move the tape to the end of the alread-recorded files like so:  
mt -f /dev/[path-to-tape] eom
  e.g.:  
mt -f /dev/st0 eom
  Now you can use tar to add a file to the tape without over-writing the existing data.