Split one external drive into two on a Mac: partitions or APFS volumes
Partitions give each part a fixed size and its own format. APFS volumes share space. Which to use, how to set up each in Disk Utility, and what erases data.
You want one drive to do two jobs: backups and files, or a Mac part and a Windows part. macOS has two ways to split a drive, and they behave differently.
| Partition | APFS volume | |
|---|---|---|
| Size | Fixed when you create it | Shares the free space, with optional limits |
| Formats | Each can be different, such as APFS and ExFAT | All APFS |
| Readable on Windows | The ExFAT part, yes | No |
| Adding one erases data? | Sometimes | Never |
Adding an APFS volume
If the drive is already APFS and only Macs use it, this is the easy way. In Disk Utility, select the existing volume and click + (Add APFS Volume). Name it, and use Size Options to set a reserve or a quota if you want limits.
Adding a partition
- Back up anything on the drive first.
- In Disk Utility choose View, Show All Devices and select the drive at the top level.
- Click Partition, then +.
- Set a name, a format and a size for the new partition. Drag the divider in the pie chart if you prefer.
- Click Apply.
Disk Utility can often shrink an existing APFS or Mac OS Extended partition without erasing it. It cannot always do so for ExFAT or FAT32, and drives using the older Master Boot Record scheme may need erasing as GUID Partition Map first.
diskutil list externalCommon setups
- Time Machine plus files: an APFS volume for each. See using part of a Time Machine drive.
- Mac plus Windows: an APFS partition and an ExFAT partition. See formatting for Mac and Windows.
Common questions
Should I use a partition or an APFS volume?
An APFS volume if only Macs use the drive, because volumes share free space. A partition if you need a different format, such as ExFAT for Windows.
Does partitioning an external drive erase it?
Not always. Disk Utility can often resize APFS and Mac OS Extended partitions. ExFAT, FAT32 and older partition schemes may need erasing first.
Can I have Time Machine and files on one drive?
Yes. Add a second APFS volume for files beside the Time Machine volume.