How to boot from an external drive that contains time machine backup?

According to the last bullet on this page, external drives with time machine backup should be bootable and appear as EFI Boot on the Startup Manager screen. My MBP is running lion 10.7.5. I formatted a new external hard drive to HFS+ encrypted by erasing the drive using disk utility, and its map scheme became GUID automatically after the erasing operation. After time machine created a full backup on the drive, disk utility even shows it's bootable on its disk info page. However I still don't see an EFI Boot option on the startup manager screen nor on the startup disk preference screen. I'm not sure if it's because my MBP is too old or I missed some important step. Any advice would be appreciated.

By the way, backup content is fully accessible in macOS Recovery so the drive itself should be fine as well.

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You’ve got a few different things here. First Lion 10.7.5 does not offer recovery services by default. You need to upgrade the system firmware OS X Recovery Disk Assistant v1.0 Apple also updated it a few times so the options may not be the same between the different releases. Newer OS-X & MacOS have additional firmware updates which are installed during the upgrade process.

Here’s the recovery options: About macOS Recovery

Now you added a wrinkle here by encrypting your drive via Disk Utility you run the risk of not being able to regain access. Right now you appear to still have access as the key which is imbedded in your internal drive. Once you reformat it you’ll loose it and then you’ll loose access to your encrypted external.

Here’s more on yore differences between Disk Utility encryption & File Vault: FileVault 2 vs Disk Utility Encryption

Basically I would recommend you carefully restore your data onto a fresh drive unencrypted for now so you don’t loose anything. Make sure your data is safe before moving forward!

OK, now what??

You’re running a very old OS which adds complication! I would recommend you upgrade to at least Mavericks. In the process your older firmware and drive utilities will get updated (you’ll need to upgrade the drive to GUID Journaled if it isn’t already running it.

I would recommend you create a USB thumb drive OS installer following this guide: How to make a bootable Mavericks install drive They also have guides for all of the newer MacOS’s as well. It will make upgrading much faster and offer you a means to boot up your drive just in case.

Now with the System’s OS upgraded you can either recover your files back onto your drive or reformat your TimeMachine drive back to the standard format and then copy over your stuff from the spare drive back over to it.

Here’s how to setup your startup drive if you want that encrypted: Use FileVault to encrypt the startup disk on your Mac & How to encrypt your Mac with FileVault 2, and why you absolutely should

To protect your external TimeMachine backup you’ll need to use a good 3rd party encryption program. This is one of the better ones: Concealer sadly Apple still does not offer external drive support with FileVault2 yet.

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Hi Dan, thanks a lot for all the suggestions. In fact I'm in the progress of upgrading from Lion to El Capitan and then possibly High Sierra. While I was backing up my data, I learned from the link above that TM drive is itself bootable. I thought that means I could work with my MBP using macOS contained in external TM drive as if it's booted from the internal one. I went ahead and tested it out, then got stuck on how to make the TM drive appear on the Startup Manager screen. Sorry I didn't quite get why I need OS X Recovery Disk Assistant. Are you suggesting that it is the tool I need to make my external TM drive bootable? But then why does disk utility mark my TM drive as bootable already? For encryption key, I have the key written down in a safe place so I'm sure I won't lose it. But maybe the disk encryption is what prevents the external TM drive from being recognized as a startup option?

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The installer USB drive was to make your life easier. I have one for each major OS release just in case! I'll have to admit I'll need to do some trimming soon as the ring is getting quite full ;-}

I guess I didn't quite make it clear. Apple has altered things enough from the first generation of the recovery firmware (within your Mac EFI) as well as the OS you are likely encountering incorrect information from the guide.

If you have the latest then the answer is true! You would be able to boot up under the TimeMachine backup but it would need to be unencrypted! Apple does not offer any means presently to encrypt a TimeMachine backup (drive wise).

I guess the easies way to think this through is the order of access. Just like how you put your sox on before you put your shoes on the encryption of the drive needs to be after the drive is accessible. So here you would need to have first installed a full and proper OS on the drive then sure around and use the drive as your TimeMachine backup as the OS is not encrypted here the drive would boot and then when you got to the TimeMachine backup (using a third party app) you would have access to it.

The other way which is convoluted would to have two partitions on this drive the first one not encrypted with your OS installed and then the second larger one for your encrypted partition with the TimMachine backup.

I still don't trust encrypted drives via Disk Utility as I've had to many issues with them.

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