New Mac systems built in 2010 and later support an Internet Recovery mode, which can be used to download the boot image generally stored on the OS X Recovery partition so you can run diagnostics.

One way of avoiding a disastrous upgrade to Apple's latest version of macOS is to partition your Mac's hard drive and install Catalina alongside your current operating system.

Though you can install Mavericks (OS X 10.9) directly from your Mac's hard drive, a bootable installer drive can be more convenient for installing the OS onto multiple Macs. And if your Mac is ...

Everything you need to know about Apple’s new MacOS 10.15 before you install it on your Mac.