Repairing a greyed-out disk on OS X
A request I recently got was to fix an external USB disk. This disk appeared to be broken beyond repair, as all options on OS X’s Disk Util were greyed out. Both on OS X and the recovery boot system. Luckily, I was able to restore the disk. Unfortunately, I was not able to recover the contents.
Unable to repair the disk with OS X, I resorted to using Windows to fix it. I download a free copy of Windows XP from Modern.IE and installed it on VirtualBox. This should be a straight forward process as Microsoft offers the Windows image as a ready-to-run appliance.
To ensure that VirtualBox has exclusive rights to your USB disk, add a filter for it. In VirtualBox, open the settings for the virtual machine. On the Ports tab, go to USB. Using the green plus button, add the USB disk that you would like to fix.
Boot the virtual machine and wait for it to finish. The USB disk should be automatically recognized if you added the filter in the previous step correctly. Open the start menu and click on Run. Type diskmgmt.msc to open the Disk Management application.
If you ran into the same problem I did, it should say GPT Protective Partition on Disk 1. While this sounds bad, it is a very easy to fix problem.
Open the start menu, click on Run and type cmd to open the command prompt. Type diskpart to run Microsoft DiskPart, a command line utility for organizing your disks. Type list disks to list all disk currently on the computer. It should return the system disk (Disk 0) and the USB disk (Disk 1). Verify this by confirming the disk size.
Type select disk 1 to select the USB disk. DiskPart will confirm your selection. Type clean to fully erase the disk. This may take a while. Afterwards, you may shutdown the virtual machine using the start menu.
After returning to OS X, you will immediately be prompted that the inserted disk was not readable by this computer. Click on Initialize… to initialize the disk.
After initializing the disk, you are able to partition it again.