When you’re installing an operating system you’ll need a boot media. Optical discs and external drivers are popular boot media.
Optical disc(CD-ROM, DVD, Blu-ray)
Optical discs as a boot media were once popular but are becoming less and less popular in favor of USB drives.
External drive/flash drive (USB/eSATA)
USB storage can be used as boot media. The USB must be bootable and the computer must support booting from USB. If those conditions are met, you’ll be able to plug in your USB into an available USB interface, start your computer, and start installing an operating system from the USB drive.
If you want to install many different types of operating systems, you can use external media. This way the device is portable and you’ll know you’ll have the operating system you need to install as you move to different compters.
Some external drive can mount an ISO image, which makes the drive look like a CD-ROM/DVD-ROM image. So you can plug your external drive in, and the computer will see it as a CD/DVD-ROM drive.
Network boot (PXE)
PXE is known as “Pixie.” It stands for Preboot eXecution Environment. PXE is used in larger scale environments. The operating system files are stored on a central server. You’d then boot your computer to use PXE. If your computer can boot using a network interface card to a PXE server, then you can use this boot method. This way you wont have to have any local installation media.
In a macOS environment, the equivalent of PXE boot is called NetBoot. NetBoot is an Apple technology that allows you to boot macOS from the network.
Internal fixed disk (HDD/SSD)
Internal hard drive (partition)
Another OS installation method is to partition your internal hard drive. You’d have one partition to boot and run the installation process and that would install the OS into a separate partition on the same physical drive.
