What external drive can be used to back up mac OSX 10.9.5?

Pretty much any kind as long as it is correctly formatted. Typically that would be GUID MacExtended journaled, not case sensitive.

Here are some links but if you're using Disk Utility on Mavericks then take note that the links are authored for much newer OS versions.

File system formats available in Disk Utility on Mac - https://support.apple.com/guide/disk-utility/file-system-formats-available-in-disk-utility-dsku19ed921c/mac

- Choose the appropriate system version to get the correct document

Partition schemes available in Disk Utility on Mac - https://support.apple.com/guide/disk-utility/partition-schemes-disk-utility-mac-dsku1c614201/mac

- Choose the appropriate system version to get the correct document

Erase and reformat a storage device in Disk Utility on Mac - https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/20.0/mac/11.0

- Choose the appropriate system version to get the correct document

A relatively inexpensive USB external storage drive can be used as backup

with Mac's own Time Machine software; the Mac can reformat the same.

Any computer store or even walmart etc, likely has something that'd do.

A 4TB storage drive w/ USB connection (included) may cost $85 - $120.

And so long as its attached by USB cable, with certain rules followed; it

can work for quite some time. ~ Read up on how to use Time Machine.

Consider once you let Time Machine in Mavericks 10.9.5 format USB drive

(HFS+ by default) the unit should just work. ~ And no additional Software

is required. This is usually why some third-party drives fail; they add SW.

• Back up your Mac with Time Machine - Apple Support

https://support.apple.com/kb/HT201250

• How to Set up and Use macOS and OS X Time Machine (Guide) - AppleToolBox

https://appletoolbox.com/how-to-setup-and-use-osx-time-machine-guide/

• Backup disks you can use with Time Machine - Apple Support

https://support.apple.com/en-us/HT202784

[I've at least one of these drives, for each of my macs; and a spare too.]

• Apple OS X and Time Machine Tips | an archive by J. Pond (2013)

http://oldtoad.net/pondini.org/OSX/Home.html

I know you would love for me to say, "Go out and buy XYZ," but I can't. For one, most drives are formatted for Windows, and I suspect the ones for Macs are increasingly being formatted for the big changes that occurred 5 years ago. You are running a 10 year old computer which if you wish to keep it running will mean you will have to learn more about its special needs.

Second, the kind of drive you buy will depend upon various factors and what I would buy may not be what you would like. For you it will depend upon the storage capacity you need which might typically be about 2-3x the size of what's inside your computer. Some people like small, portable drives which I do not like because I don't know which brand drive is inside and they often depend upon the computer for power.

You can buy pretty much any drive and don't be put off by the "10.12" business because that's likely referring to bundled software which you do not need. You will, however, very likely need to reformat the drive which is what all those links were about. However, the layouts of the menu in Disk Utility as pictured in those support articles has changed and Apple only refers to how it currently looks, not how it looked 10 years ago which is what you need. many things are similar but in order to do this you need to understand what you are doing. I would say that goes in general for owning a computer with a 10 year old operating system version. You are living in a different era and you are going to have to be your own computer support person. I too run Mavericks and you really need to learn how to use a Mavericks machine in 2021. You won't be able to follow those instructions like cookbook because they are written for a newer version of Disk Utility. They are very similar though, but understanding the differences will need a bit of work on your part so you know what is going on and what to look for when "press X" isn't there but somewhere else in the menu.

This is like asking which car should you buy. I could recommend something that I would buy but you wouldn't like it because it isn't portable and it takes somebody who is comfortable doing all the things you do not want to learn to set it up. Mind you, it is the least expensive solution and I know that I am getting quality parts. I can also use multiple backup drives with it so if somebody steals my backup drive and computer I have another backup drive in the basement and still have my stuff.

I suggest you browse this site until you find something that meets your needs:

https://www.owcdigital.com