Erasing a USB drive destroys all data on it. Confirm the correct disk before clicking Erase — bootable installer sticks often show two partitions that look similar in Disk Utility.
What you will achieve
Safely unmount and erase a macOS installer USB so you can reuse the stick for normal storage or prepare a fresh bootable installer without leftover hidden partitions causing confusion.
Creating a bootable installer reformats the drive with GUID partition map and often leaves EFI and install partitions. A proper erase returns the stick to a single usable volume.
1) Eject before physical removal
- Click the eject icon next to the USB in Finder sidebar, or drag the volume to the Bin.
- Wait until the icon disappears before unplugging — yanking a stick mid-write can corrupt the partition map.
2) Open Disk Utility
Launch Applications → Utilities → Disk Utility, or search Spotlight for Disk Utility. On macOS Ventura and later, choose View → Show All Devices from the menu bar so you see the physical disk, not just the top-level volume.
3) Select the physical USB disk
Click the top-level device entry (e.g. “SanDisk Ultra 32GB”) — not the nested “Install macOS Sonoma” volume beneath it. Selecting the wrong level can leave orphan partitions.
4) Erase with correct scheme
- Click Erase.
- Name: any label you prefer (e.g. “USB Storage”).
- Format: APFS for macOS-only use on 10.13+; ExFAT if you need Windows compatibility.
- Scheme: GUID Partition Map — required for future macOS installers and Apple Silicon boot.
- Click Erase and wait for completion.
Intel Mac installers historically used Mac OS Extended (Journaled) for older macOS versions. Modern installers (Big Sur onward) require APFS on GUID. Apple Silicon Macs will not boot from MBR-partitioned USB sticks.
5) Verify partition layout
After erase, Disk Utility should show one container and one volume under the physical disk. If you still see “EFI” plus a stray “Install” volume, select the physical disk again and erase once more with GUID + APFS.
6) Reuse for a new installer
When you next need a bootable USB, follow your create-installer guide with createinstallmedia or the appropriate method for your target macOS version. The stick must be at least 16 GB for recent macOS releases; 32 GB is safer for Sequoia-era installers.
7) Troubleshooting stubborn sticks
If Erase fails with “Couldn’t unmount disk”, quit any app accessing the USB (including Finder windows open on the volume). Run First Aid on the physical disk. On rare occasions, boot into Recovery and erase from there — useful when the stick was left in a half-written state during installer creation.
Verify
Disk Utility shows a single APFS or ExFAT volume under GUID on the physical USB, with usable capacity matching the stick size minus small overhead.