Make a Bootable macOS Sierra USB Installer

A bootable macOS Sierra USB installer lets you install or reinstall Sierra on a compatible Mac, including when its internal startup disk is unavailable. You’ll need the Sierra installer, a USB drive that can be erased, and access to a Mac that can run the installer-making command.

What You’ll Need

A USB flash drive with at least 16 GB of capacity. The process erases everything on it, so copy off any files you want to keep. A larger drive is fine.
The macOS Sierra installer app. It should be named Install macOS Sierra.app and be in the /Applications folder. Sierra is an older macOS release, so Apple’s current App Store pages may not offer it on every Mac. Get the installer through Apple’s official download or support resources when available; avoid unofficial installer packages.
A Mac and administrator access. You’ll use Terminal and enter an administrator password. The Mac must be able to download or already have the installer and must be compatible with the tools needed to create the media.
A stable power connection and internet access, if needed. Creating the drive is local once the installer is present, but downloading it and some installation steps require internet.

Step-by-Step

Check that the installer is complete. Open Finder, choose Applications, and look for Install macOS Sierra.app. If it is missing, download Sierra from an Apple-provided source compatible with your Mac. If the installer opens automatically, quit it without starting an installation. Do not move or rename it; the command below expects its standard name and location.
Connect the USB drive and identify it. Open Disk Utility from Applications > Utilities. Select the physical USB device in the sidebar, not just a volume beneath it. Confirm that it is the drive you intend to erase. If needed, choose View > Show All Devices so the device appears. Write down its name, but do not assume that a particular disk identifier belongs to it.
Erase and format the USB drive. With the physical device selected, click Erase. Give it a simple name such as SierraUSB. Choose Mac OS Extended (Journaled) as the format and GUID Partition Map as the scheme if Disk Utility offers a scheme selector. Click Erase and wait for it to finish. This permanently deletes the contents of the selected drive. If Disk Utility will not let you select a scheme at the device level, use the volume and proceed; the installer command can prepare the destination volume.
Open Terminal. Go to Applications > Utilities > Terminal. Enter the following command on one line, changing SierraUSB only if you chose a different name for the USB volume:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/SierraUSB --applicationpath /Applications/Install\ macOS\ Sierra.app

This is Apple’s Sierra-era createinstallmedia command format. The installer app and USB volume names in the command must match your actual files and volume. Do not run a command copied from a source you do not trust.

Authorize the command. Press Return. When prompted, type your Mac administrator password and press Return again. Nothing appears on screen while you type the password; that is normal. If Terminal asks to erase the volume, check the volume name carefully, then type Y and press Return to confirm.
Wait for the media to be created. Terminal reports progress while it erases and copies installer files. Keep the drive connected and do not quit Terminal or put the Mac to sleep. The process can take some time. Wait until Terminal reports that the install media is available and returns to the command prompt.
Start the target Mac from the USB drive. Leave the installer connected and shut down the Mac you want to install Sierra on. Turn it on and immediately hold Option (⌥). Release the key when Startup Manager appears, then choose the installer, often labeled Install macOS Sierra, and press Return. On an Intel Mac with a firmware password, Startup Manager may require that password.
Install Sierra or use the recovery tools. After the installer loads, choose the language if asked. You can start the installation or open Disk Utility from the Utilities menu to inspect or repair disks. Before erasing or repartitioning an internal disk, confirm you have a backup and have selected the correct disk; those actions can destroy data. Follow the installer’s prompts to continue.

Common Mistakes

Using the wrong volume name. The name after /Volumes/ must match the mounted USB volume exactly, including spaces and capitalization. If the name has spaces, escape each space with a backslash or put the full path in quotes.
Leaving the installer outside Applications. If the app is in Downloads or has a different name, the command will not find it. Move the genuine installer app into Applications or adjust the command carefully to its actual path.
Selecting the wrong drive in Disk Utility. Check the device name and capacity before erasing. Never choose your Mac’s internal disk by mistake.
Using a damaged or undersized drive. A drive with less than 16 GB, connection problems, or failing flash memory may not work reliably. Try a known-good drive if creation stalls or errors recur.
Expecting every Mac to support Sierra. A bootable installer does not make an incompatible Mac compatible. Check the target Mac’s model compatibility with macOS Sierra before starting.

Tips

Use a short USB volume name without spaces to make the Terminal command easier to enter.
Check the command before pressing Return, especially the two paths and the volume name. The command’s --volume destination is erased.
Keep the installer drive somewhere safe and label it with its macOS version. It is also useful to keep a separate current backup; installation media is not a backup of your files.
On many Intel Macs, Option at startup opens Startup Manager. Startup-key behavior can vary with keyboard type and Mac configuration; use a wired keyboard if a wireless one is not detected early enough.

Troubleshooting

Terminal says the command or file does not exist: Verify the app is named Install macOS Sierra.app and is directly in Applications. Confirm that the command was entered as one line, with spaces escaped as shown.
Terminal cannot find the volume: In Finder, check the USB drive’s displayed name. If necessary, run ls /Volumes in Terminal to see mounted volume names, then correct the command. Reconnect the drive if it is absent.
The command reports that the installer is invalid or damaged: The download may be incomplete, altered, or incompatible with the Mac running the command. Delete the incomplete copy and obtain the installer again from an Apple-provided source. Check the Mac’s date and time if certificate-related validation fails, and avoid changing security settings to bypass a warning.
The USB installer does not appear at startup: Recreate it after checking that the command completed successfully. Try another USB port, disconnect other external storage, and hold Option immediately after turning on an Intel Mac. If the target Mac is not compatible with Sierra, the installer may not be usable there.
Installation fails after booting: Check compatibility, available disk space, and the target disk’s condition. If installation needs internet access, connect to a reliable network. Back up important data before using Disk Utility to erase or reformat any internal drive.

Frequently Asked Questions

Will making the USB erase its existing files?
Yes. The drive is erased during setup, so copy its contents elsewhere first.

Can I use a 8 GB USB drive?
No. Use a drive with at least 16 GB; a larger drive is acceptable, but its contents will still be erased.

Can I create the installer on a Windows PC?
This guide uses Apple’s macOS installer app and Terminal command, so you need access to a Mac to follow these steps.

Does the USB installer include a license or guarantee compatibility?
No. It is installation media, not a license or compatibility check. The target Mac must support Sierra, and you should use macOS in accordance with Apple’s terms.

How do I install Sierra on an Intel Mac?
Connect the USB drive, start the Mac while holding Option, select the installer, and follow the onscreen instructions.

Can I reuse the USB drive afterward?
Yes. Once you no longer need the installer, erase and reformat the drive in Disk Utility before using it for ordinary storage.

General steps for current mainstream versions; menus can differ by device and version. See the disclaimer or report an error.

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