how do I backup mi iMac and my MacBook Air using the same external drive?

Hello bsheerz,

Welcome to Apple Support Communities!

We understand you want to know if you can back up both your Macs using the same external drive. We're happy to help!

You can backup both your Macs using the same external drive if you wish. We are providing you with the article for backing up your Macs using Time Machine.

Back up your Mac with Time Machine

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Although you can have two different backups on one external drive, my personal opinion is that you can avoid any possible glitches by simply partitioning the external into two partitions. Name each one for the device it contains ("iMac backup" and "MBA bakcup") and you will get two icons on your desktop. That way, they each have their "own space".

Having said all of that, I need to add: it is very bad practice to have all of your backups on the same drive. If the drive fails (and they all will sooner or later), you will have lost all the backups. I have 5 different external drives and at least 2 backups of everything I have on different drives.

Not a very wise decision, why.... simple! All hard disks fail, when is the magic question and is impossible to answer. When the drive dies it would die with backups to 2 computers, why would you risk that to save a few $$. Do yourself a BIG favor and buy a high quality EHD for the 2nd computer and use it for TM.

Experienced users on these forums recommend https://eshop.macsales.com/shop/owc-mercury-elite-pro

Time Machine backs up everything on the computer. The first back up it does is to backup the whole drive. All backups after the first one are for all updated or changed files. That means if you change a file (say for the sake of argument an Excel file) it will save the updated version and also have the original version of the Excel file too. In addition, if you upgrade Mac OS it will have both the original version from the original TM backup and the updated version(s).

Your welcome.