Use a Single External Hard Drive for Time Machine Backups and File Storage

Having regular backups of your Mac is a necessity, and there is really no easier way to consistently back up your Mac than by using the excellent Time Machine feature of OS X. But with the enormous size of external hard drives and their prices becoming cheaper and cheaper, it’s not always necessary to dedicate an entire gigantic hard disk just for Time Machine backups, particularly if your Mac has a smaller hard drive and thus the backups won’t take up that much space in general. For these situations, configuring the single external hard disk to have dual use is an excellent choice. The end result will be an external storage drive split into two partitions, one to be setup exclusively for Time Machine backups, and another partition intended for typical file system access and file storage.

The basic process may be familiar to Mac users who have setup drive partitioning and backups before, but we’ll cover every step to be sure everything is configured correctly.

Requirements

• Any Mac running OS X with Time Machine support (every modern version)
• Large external hard drive (check out this Amazon deal)
• Minor patience, and about 10 minutes for initial setup

Note on buying external hard drives: it’s almost always cheaper to buy a generic external hard drive and format it yourself to be Mac compatible. Drives that are pre-formatted for OS X are usually no different than a standard external drive, other than having a higher price tag.

Step 1: Format the Drive to “Mac OS Extended” Compatibility

The first set of steps involves formatting the drive. You can partition a drive without formatting, but we’ll cover this process anyway because many third party hard drives ship with Windows-centric FAT32 or NTFS file systems which, while they are compatible for dual use with both Mac and Windows, are not compatible for using as a Time Machine drive, and as they are not exclusively formatted for the Mac, will have other limitations which are not desirable for exclusive Mac OS X usage.

This process will erase all data on the hard drive, meaning this is best to pursue when you first get a new external drive for backups and file storage.

• Connect the external hard drive to the Mac
• Launch Disk Utility, found in /Applications/Utilities/
• Select the external hard drive from the drive list on the left, then click the “Erase” tab
• Choose “Mac OS Extended (Journaled)” as the format type, ignore the naming convention for now, then click “Erase” and confirm the drive will be erased
  

How long it takes to format a drive depends on a variety of factors, including drive speed, interface speed, and total disk size. Just let the process go, don’t be surprised if it takes a few minutes.

Step 2: Create Two Partitions for Time Machine & Storage

Next we’ll set up the external hard disk to have two separate partitions, one for the Time Machine backups and the other for regular file system access.

A quick note about sizing: It is good practice to set the Time Machine drive to be at least 2x-3x your primary hard disk size. For example, if the Mac has a built-in 128GB SSD drive, setting the Time Machine partition to be at least 384GB or larger would be ideal. You can certainly get away with smaller sizes, but because Time Machine takes incremental snapshots of the data on your Mac, the backups will simply capture more data for a longer period of time if the partition size is larger. To be clear, backups will not stop once the maximum space is reached, it will simply rewrite older backups, thus preventing access to old drive states as they become rewritten. We’re going to use an even 50/50 partition scheme for this example (specifically, a 1.5TB drive split into two 750GB parts) though you can configure yours as appropriate.

• When the drive has finished formatting, choose the “Partition” tab
• Pull down the “Partition Layout” menu and select “2 Partitions” to split the drive into two equal partition sizes divided 50/50
• Adjust the partition size allocation if desired by dragging the boxes to adjust size, or by manually selecting a partition and entering a desired allocation in the “Size” input box
• Name the two partitions accordingly, select the first partition and name it something like “Time Machine Backup”, then select the other partition and name it something like “File Storage”
• Choose “Apply”, then confirm the changes by clicking “Partition” when asked
  

Partitioning a drive can take a few minutes, depending on the total capacity of the disk. Once that process is finished you can quit out of Disk Utility.

Step 3: Set Time Machine to Backup to a Specific Partition

With the most technical aspects now finished, you can specify the partition to become the Time Machine backup. This will also initiate the first backup of the entire Mac with Time Machine, which is usually the lengthiest backup since it’s going to back up every single thing.

• Go to “System Preferences” from the  Apple menu and then choose “Time Machine”
• Click the “Select Disk” button and let the list populate
• Choose the partition named “Time Machine Backup” from the list, then confirm the choice by clicking “Use Backup Disk”
• Let Time Machine backup for the first time

While you’re in the Time Machine settings, you can choose to encrypt the backups by checking the appropriate box (yes, you can encrypt them later if you change your mind), and you can also exclude files or folders from the backups through simple drag and drop specification by way of the “Options” button if desired. The default configuration remains unencrypted and excludes nothing, which is satisfactory for many use cases.

Again, the first initial backup process will take quite a while since the entire Mac is being backed up. Let the entire process run through its course, this may be best done overnight if the primary Mac hard drive is enormous since it can several hours to perform the initial backup. Backups performed after the initial sequence will be much faster and smaller, because they will be delta backups, focusing on files that have been added, deleted, or changed from the Mac, rather than just copying the entire drive and it’s untouched contents over and over again.

All done! Easy backups and access to classic file storage are good to go

Now that everything is setup you will have one partition automatically serving as the backup drive, and the other accessible as usual through the file system for general file storage of things like movies, large video collections, pictures, media, downloads, or whatever else. How to differentiate between the two drives? Other than the obvious name differences that were specified during configuration, you’ll discover the icons serve as an indicator of which partition/drive does what purpose. The normal file system storage partition will have a standard orange external drive icon, and the Time Machine partition will have a green icon with the backup logo on it.

Accessing the standard file system partition is done through any Finder window, where it will appear in the sidebar under “Devices”, or if you have drive icons set to show up on the desktop, it will appear there.