The Startup Manager, available in iBook, Power Mac G4 (AGP Graphics), G5, PowerBook (FireWire (with a rear that looks like if it was black), and otherwise a silvery titanium or aluminum colored), and slot-loading iMacs G3s, G4s, G5s, Intel Macs came in two different flavors.
If your Mac was PowerPC, and you pressed Option key of a wired Apple keyboard, the screen looked like:
You would select the volume, and click on the right hand pointing arrow to boot from that volume. If you clicked on the curly
arrow, it refreshed the screen with new bootable volumes if you chose to add some in the meantime. This worked with USB, Parallel ATA, and Firewire bootable volumes on Mac OS 9, and Firewire, Parallel ATA, and SATA bootable volumes on Mac OS X.
If your Mac was Intel, it looked like:
selecting the volume and clicking on the arrow would boot the volume in question.
This worked on Intel Macs with USB, Firewire, Parallel, ATA, or SATA volumes.
Note, if the operating system on the volume was too new or too old for the Mac in question, the Startup Manager may not be able to detect that, or if there is something else damaged on the said volume, it may fail to boot. Frequently the Startup Manager would work when the 'C' key booting failed.
If the machine is over 4 years old, the Startup Manager may fail due to an old PRAM battery. Only the iBook G3 didn't have a PRAM battery. All others described above do, and may need an authorized service technician to replace.