Snow Leopard: This Disk Cannot Be Used to Start Up Your Computer

Submitted by Bill St. Clair on Sat, 29 Aug 2009 03:06:25 GMT  <== Computers ==> 

My new Mac OS X Snow Leopard disk was delivered today by FedEx. After my work day, and the completion of a backup, I rushed to install it. The icon for my boot disk had an exclamation mark on it, and when I clicked on it, it said "Mac OS X cannot be installed on ..., because this disk cannot be used to start up your computer."

This disk cannot be used to start up your computer

I almost went to the store to buy another disk, but some time with Google found the solution in this post from William Maxwell:

For all affected users: Please open Disk Utility > Highlight the boot HD > click on the partition tab. Just move the partition size a bit and return to original value. Then hit apply. Then see if the install works. Thanks.

He clarifies with:

Don't hit the plus button. Just grab the little handle at the bottom right of the partition table and drag it up a bit. Then drag it back down. Then hit apply. I want to see if this tickle of the partition table addresses the issue. Thanks.

Some people also had success with uninstalling PGP Desktop. That didn't work for me, but the fix above did.

(I was able to successfully reinstall PGP Desktop after my Snow Leopard install, but I'll refrain from using PGP Disk until they ship a certified-Snow-Leopard-compatible update).

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Comments (3):

PGP is responsible

Submitted by Bill St. Clair on Sat, 29 Aug 2009 03:33:12 GMT

According to "dre^" on the #macosx channel at, PGP munges the partition table, and changing the partition size (smaller and back) rewrites it, fixing the problem. I took the screen shot above AFTER my install, but after reinstalling PGP, so the problem is back, and I may encounter it again when I do my next install.

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Disk not recognised by Snow Leopard installer

Submitted by PeterG on Sat, 05 Sep 2009 11:57:22 GMT

I had the same problem: "Mac OS X cannot be installed on 'HD' because this disk cannot be used to start up your computer".

I uninstalled PGP Desktop, did a disk permissions repair and rebooted.
Snow Leopard then installed without a hitch.

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After uninstalling PGP ...

Submitted by PGP Anonymous on Thu, 15 Oct 2009 17:10:02 GMT

After uninstalling PGP, rebooting will usually complete the process allowing the system installer to work.

The PGP uninstaller does not force a reboot as for the most part it is not required. In this case, a reboot allows the system to re-read the partition table that has been changed so that the installer will be able to work properly.

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