Recovery disc
The terms Recovery disc (or Disk), Rescue Disk/Disc and Emergency Disk all refer to a capability to boot from an external device, possibly a thumb drive, that includes a self-running operating system: the ability to be a boot disk/Disc that runs independent of an internal hard drive that may be failing, or for some other reason is not the operating system to be run. The focus of recovery or rescue is not to lose the data files on the hard drive; the focus of restore is to restore the operating system's functionality (and subsequently restore the contents of one's latest backups).