You can connect an external drive via USB or Firewire (if the computer still has FW). Does the external have a bootable OS installed? If so, restart and press Option as soon as you hear the chime. Continue holding Option until you see a choice of startup disks. At this point, click the external's icon (assuming that it shows) and press Return. It may take longer to boot from the external, so be patient.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365