One long-standing and useful feature in OS X is its ability to boot from practically any mountable partition, including external drives. This allows for quick ways to test an installation, isolate different uses of your Mac to separate boot drives, troubleshoot aspects of your Mac’s hardware and internal storage, and create bootable backups of your Mac’s hard drive. While useful, there are times when your Mac will not boot to such partitions, even if they are perfectly healthy. If so, then here is what to do about it.

1. OS X version mismatch

First check if the version of OS X on the external drive supports your current Mac. You might have created the secondary boot drive using a prior Mac running older OS X software, and if so, then this may not boot properly (if at all) when used on newer Mac hardware. For full compatibility, be sure you have the OS X version that shipped with your Mac (or later) installed on the drive. To do this, you should be able to perform a reinstall of OS X onto the drive using the OS X Recovery utility without wiping it. As with a standard reinstallation, this should only replace and update the system software on the drive, and not affect any of your data on it.

2. Firmware password enabled

When you boot OS X, you should be able to load the boot menu by holding the Option key when you hear the boot chimes; however, if you have a firmware password enabled then you may not see this menu and instead may see a password prompt. You will otherwise not be able to instruct your system to load the alternative boot drive at startup. To overcome this, you will need to either supply your firmware password or disable it. On systems made before 2010, a hardware change such as removing RAM modules was enough to reset the firmware password, but in newer systems, you will need to contact Apple Support and have this done in person at an Apple Store.

3. Wireless or third-party keyboard

If you use a wireless keyboard, then any keys you press will not be recognized if they are held down before the system detects the keyboard. This is done when the boot chimes sound, so be sure to press and hold the Option key or other startup argument key combinations immediately when you hear the boot chimes, and not before. This requirement does not apply to USB keyboards, so you might also consider forgoing your wireless keyboard for a USB one when attempting these alternative boot options.

In some cases, third-party wireless and USB keyboards may not be recognized by the system at boot, which will likewise prevent any key combinations from being recognized at startup. In these cases, try using another keyboard (specifically the one Apple provided with your system).

4. Conflicting peripheral devices

While less of a problem, you may have an issue with other external devices that your Mac is connected to, which may prevent your hard drive from being recognized. This is especially true if you are using a USB hub or extension cable. To overcome these issues, try disconnecting all devices from your Mac and attach your drive directly to your system. Also ensure you use a provided power source for the drive, if available, as this will ensure it is receiving ample power and not relying solely on the USB connection for power.

5. Manually select in the Startup Disk system preferences

An alternative to using startup options for loading a secondary boot partition is to select your desired drive in the Startup Disk system preferences. This approach can be done even if you have a firmware password enabled on your Mac, and is the way to permanently change the boot drive on your system so you will not have to use special keys at startup.

6. Check the boot drive’s partition map and formatting

A final issue could simply be that your desired boot drive has become damaged. For OS X to load, the drive must have a healthy GUID partition scheme, and be formatted to HFS+ (aka, “Mac OS X Extended). To check this, open Disk Utility and select your drive device in the sidebar. Then press Command-i to get information on the drive, where you should see in formation about its partitioning. You can do the same for the boot volume listed under the drive, where you should see its format listed.

If you are using FileVault full-disk encryption on the drive, then it will appear as a Logical Volume Group and not show you the partitioning scheme. This should mean that the drive is already using GUID partitioning, as it is a requirement for CoreStorage (the technology used for creating the logical volume group), but you can also check by opening the Terminal and running the following command:

diskutil list

The output of this command will list all physical and logical drives available to your system, where you will see those that are labeled as Apple_CoreStorage partitions will also have GUID_partition_scheme listed.

7. Wipe and reinstall to that partition

Lastly, if you are not relying on any of the drive’s data and only wish to have a secondary boot drive for troubleshooting purposes, then you can use Disk Utility to wipe the partition and then reinstall OS X to it from scratch. Doing so should ensure the drive’s logical environment (software and storage setup) are ready for booting, so if the system still refuses to boot it, then this suggests a hardware incompatibility that is preventing the drive from being available at bootup.