To use one external drive for both Time Machine backups and everyday files on a Mac, give each job its own volume. For a drive you can erase, format it as APFS, then add a second APFS volume in Disk Utility. Choose one volume for Time Machine and keep your documents on the other. This separates their contents, but both volumes still rely on the same physical drive.

Before you set up the drive

First, check whether the drive already contains files or Time Machine backups you need. Erasing a storage device permanently removes its data, so copy anything important elsewhere before you start. If the drive already holds data and you don’t have a separate copy, stop before changing its format or partitions.

APFS is the practical choice for a Mac-only drive: Apple recommends APFS or APFS Encrypted for Time Machine disks, and APFS volumes share free space within one container. Apple also advises creating multiple APFS volumes instead of partitioning an APFS disk in most cases. Apple’s Time Machine disk guidance and Disk Utility’s APFS volume guidance describe these options.

Set up separate volumes in Disk Utility

Connect the drive, open Disk Utility, and choose View > Show All Devices. If the disk is new or you’re ready to erase it, select the physical drive at the top of the sidebar and click Erase. Choose GUID Partition Map for the scheme and APFS or APFS (Encrypted) for the format, then confirm. The Erase control removes data from the selected device; double-check its name and capacity before proceeding. Apple documents these erase steps and identifies GUID Partition Map as the scheme for Intel and Apple silicon Macs. Initialize an external storage device and partition schemes in Disk Utility.

If the disk is already APFS and has an APFS container, you can usually add a volume without erasing the container: select an existing APFS volume in Disk Utility and click Add Volume. Name the new volumes something recognizable, such as Time Machine and Files. If the disk is formatted another way, don’t assume that adding an APFS volume is available; check Disk Utility’s displayed format and Apple’s instructions before changing it. Apple’s steps for adding an APFS volume.

Select the backup volume in Time Machine

Open Apple menu > System Settings > General > Time Machine, click Add Backup Disk, select the volume named Time Machine, and click Set Up Disk. If macOS offers encryption or a backup space limit, choose the settings you want and keep the password somewhere safe. Apple notes that the available options depend on whether the disk supports them. Connect a new backup disk to your Mac.

After setup, save ordinary files to the Files volume in Finder. Avoid using the backup volume as general-purpose storage: separate names reduce mix-ups, and keeping routine files off it preserves its role as the backup destination. Time Machine may skip a scheduled backup when the disk is unavailable; backups resume when the Mac and disk are available again, according to Apple’s setup guidance.

Understand the shared-space and reliability limits

APFS volumes in one container share available space by default. A large file copied to Files therefore reduces space available to Time Machine, and growing backups can use space that might otherwise have gone to documents. You can set APFS reserve or quota sizes in Disk Utility’s Size Options when adding a volume, but those controls only manage space allocation; choose limits with your expected backup and file growth in mind. Apple explains shared space and APFS size options.

Two volumes are organizational boundaries, not separate hardware. If the external drive fails, is lost, or is disconnected when needed, both the everyday files and the Time Machine backup on it may be unavailable. This follows from storing both sets of data on the same physical device. If a file matters, keep another copy on a different device or location; a Time Machine volume on the same drive does not protect the Files volume from failure of that drive.

A single shared drive suits a Mac user who wants one connected device and understands its capacity and reliability limits. If the drive must also work with Windows, APFS is not the cross-platform choice; Apple lists ExFAT as Windows-compatible, while its Time Machine guidance calls for a Mac format. Consider separate storage for those conflicting needs instead of expecting one format to satisfy both.

Sources