Disk Utility: Unable to Erase or Partition a Disk after Installing OS X Yosemite

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If you try to resize or erase a partition on your disk in Disk Utility, but find the Erase tab and Partition tab grayed out or missing, it may be due to some changes made in OS X Yosemite (10.10).

When setting up your computer for the first time after installing Yosemite, one screen will ask if you would like to enable FileVault. Some users may enable FileVault without fully understanding the scope of its functions. If you disable FileVault and restart your Mac, you should be able to modify your disk in Disk Utility again.

NOTE: Disks are reformatted from Mac OS X Extended to a Logical Volume Group when upgraded to OS X Yosemite. This, in and of itself, will not impact the ability to modify the disk. You still cannot resize it, however.

Disabling FileVault

First, open System Preferences. Click on the "Security & Privacy" pane.

Next, select the FileVault tab (second from the left). You will first need to click the lock icon in the lower left of the window and enter your administrator password to be allowed to make changes. After that, you will be able to click "Turn Off FileVault."

NOTE: Sometimes, after initiating the decryption process, it will freeze with a blue bar saying "Estimating time remaining" in the System Preferences window. Restarting usually fixes this issue, and the FileVault menu in System Preferences should then have a progress bar with the actual remaining decryption time.

After the process is complete, the FileVault menu in System Preferences will have a button reading "Turn On FileVault." This is how you know it is done (you don't need/want to turn it back on). Restart again, and then you should be able to go into Disk Utility and make the modifications you want, as the disk is no longer encrypted by FileVault.

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