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Encrypting external USB bootable backup drives with Disk Utilities

For the past several years, I’ve maintained (using the excellent SuperDuper!) two bootable backups for my Mac, providing immediate, redundant recovery in case of the loss of the Mac’s internal drive. And also for the past several years, these bootable backups have taken the form of small (usually Western Digital) 2.5″ bus-power USB drives.

Since upgrading to Lion, I’ve also been happy to be able to encrypt these external drives using Apple’s Disk Utilities, allowing me to keep one disk at the office, and one at home. (Break-ins and theft are a major problem here in Spain, so knowing my backup disk at work is encrypted gives me piece of mind.)

Upgrading my MacBook Air

Recently I upgraded from a 2011 to 2012 11″ MacBook Air, and as part of that upgrade, also needed to upgrade my external bootable drives; the old ones were 256GB, matching the internal SSD of the 2011 Air, while I’d need 500GB drives to match the internal SSD of the new Air.

Can’t encrypt the G-Drive Slim

So off I went shopping to the official Apple Store in Marbella, where I was excited to see that they’d lowered the price of the tiny 500GB G-Drive Slim to 69 €. I bought two.

I ended up having to return them, though, when I discovered that Disk Utilities couldn’t encrypt the drives, failing with the following error:

MediaKit reports block size error, usually caused by not being a multiple of 512.

I emailed G-Technology support, asking if they knew why I was getting that error when trying to encrypt the drive with Disk Utilities, and got this goofy reply from Gene Gilbert:

Thanks for your email. The easiest way to do protected files on the drive is to create a Disk Image in Disk Utility, then you can add files to the disk image, and will require a password to open. The other option is to use File Vault, but that does the whole system. Other than that, it would require a third-party solution.

I replied asking if they’d mind addressing the question I actually asked, and didn’t hear back.

Apple don’t sell USB 3 drives?

The Apple Store staff allowed me to return the drives (but only after first assuming I know nothing of what I’m talking about. Took a few minutes to clear that up.) I then purchased the tried and true Western Digital Passport for Mac 2.5″ drives.

Got home, unpacked them, connected to the new Air, encrypted, and started backing up.

Waiting for SuperDuper! to do its thing, I surfed on over to the Tech Specs page at Apple, to learn more about the new Air I’d bought. (I know what you’re thinking…) Then I see it:

USB 3 ports

I look over at the Western Digital box, and see “USB 2”. Darn.

So, back off to Apple I go, to figure out how I’d missed the USB 3 drives, and there I discover they have no USB 3 drives. Maybe they’re so new…

But then back home, visiting, I get the impression that outside the world of Apple, there’s nothing but USB 3 drives! So back on the scooter I jump, and scoot on up to FNAC, where I discover the exact same Western Digital drives, for the exact same price, but with USB 3!

I bought two, having gotten the agreement from Apple that I can return their’s with no other reason than that I found a better option somewhere else.

Disk Utilities crashes when encrypting the drives

I get back home with the new USB 3 Western Digital drives, let out a small sigh when I see they have yet another variation of USB connector, hooked them up to the Air, and fired up Disk Utilities.

I selected “Mac OS Extended (Journaled, Encrypted)” and started the format process. About the time it read, “Waiting for logical volume to mount” or something like that… crash.

Started it again, tried to format. Crash.

Oddly, though, it did seem as if the encrypted volume was actually created and mounted. So I ran the “Verify” process, which… failed.

Oh crap, I’m thinking. Another drive that can’t be encrypted.

But before returning these drives, and becoming the most famous drive-returning dude in Marbella, I decided to just see if I could backup, and then boot from the thing.

So I ran SuperDuper!, rebooted, and … success!

Curious, I returned to Disk Utilities, and ran the “Verify” command on my encrypted external Time Machine drive, and one of the old USB 2 drives. In both cases, it returned the exact same verification failure.

So while I still don’t know why Disk Utilities crashes when encrypting these new disks, the verification error is apparently not evidence of Disk Utilities actually failing to format the drives.

I decided to blog about this, in case others run into this situation (since Google came up empty when I looked.)

Published inTechnology


  1. J J


    I’ve had the exact same crash happen to me repeatedly, but I’m getting a “The volume {VOLUME} appears to be OK.” message when I run Verify Disk.

    Have you had any issues with the drive since this format?

    Thanks for the post!

  2. Phil Phil

    I’m having the same problem with a USB 3.0 Seagate FreeAgent Drive; Disk Utility crashes every time I try to format it as an encrypted volume.

  3. Phil Phil

    One way around this is to do the process from the command line. Open up terminal and type “diskutil list” and find the volume that you want to encrypt. For me it was disk1s2. Then, start the encryption process with “diskutil cs convert disk1s2 -passphrase”. The process can be monitored by typing “diskutil cs list”, and looking at the “Size (Converted):” section. Even over USB 3, this will take a LONG time on a multi-terabyte volume.

  4. RBB RBB

    Hi Matt–I got that very same “MediaKit” error when trying to back up my MacBookAir. So if I understand you correctly, the DISK is NOT defective? I’m hoping not, as I threw away the darn box already. Thanks.

    • Matt Henderson Matt Henderson

      At least in my case, the disk wasn’t defective; it was simply a disk that (for whatever reason) Apple’s utility couldn’t encrypt. (I had actually bought two of the units, and got the same error on both.)

  5. RBB RBB

    Got it, thanks. And this third-party utility (SuperDuper) solved the problem?

  6. Matt Henderson Matt Henderson

    @RBB, oh, yes, sorry — in my previous reply, I’d meant that the disk was actually fine. Backing up to it using SuperDuper and then rebooting from the external drive confirmed it.

  7. RBB RBB

    Ahh, got it. Okay this may be getting too deep for me. Might just do it sans encryption.

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