I have a late 2013 21.5" iMac - Want to use external drive

You can use an external SSD as a startup drive for that iMac.

Community user @Jack-19 has posted this great user tip that lays out the how-to:

Use an external SSD as your startup disk … - Apple Community

Mcgoo4 wrote:

I want to add an external SSD drive to my old late 2013 iMac that I use in my hobby room to run my laser, stream TV, and do research. If I do this, and make that external drive my boot drive, making the old computer basically a monitor, will I be able to upgrade to the latest OS?

Adding an external SSD to your Late 2013 iMac

• will not "make the old computer basically a monitor"
• will not let you upgrade to the latest version of macOS.

The big advantage of solid state drives (SSDs) over mechanical hard drives (HDDs) is that SSDs are much faster at random access – or jumping around from place to place. When a computer jumps around, a mechanical hard drive has to move read/write heads to a new location, and then wait until the drive's motor spins the desired data around under them.

When your computer

• Starts up
• Launches an application

it does a lot of jumping around.

That's why you want a SSD as a startup drive, if your computer can take advantage of one.

Mcgoo4 wrote:

I have trouble with streaming (ESPN will stream, just no sound, has to to with OS).

ESPN – ESPN+ Supported Devices

ESPN – Computer and Browser Requirements for ESPN+

"ESPN+ supports the following computers and browsers. For the best experience, we highly recommend using the latest browser version available.

…

• Safari 14+ is supported on macOS (Big Sur 11) and later
• Chrome 125+ is supported on macOS (Big Sur 11) and later
• Firefox 128+ is supported on macOS (Catalina 10.5) and later" [sic – they should have written 10.15]

Catalina isn't recent enough to run the recommended versions of Safari. It is enough to run the current version of Firefox. If you are using Safari to stream ESPN and are having problems, I would suggest downloading Firefox and seeing whether it handles ESPN any better.

No. A computer can't boot from an OS on an external drive that it wouldn't be able to from the internal one.

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Yes.

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