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Saturday, January 17, 2009

GPT protective partition after moving a removable disk from a zpool to Windows

In an emergency, I needed an USB disk to backup some data from a Windows XP PC and the only disk I could take was a Lacie disk used as a cache device on a zpool. No problem removing it from the pool and formatting it again, thought I.

Problem is that when I plug the disk on the troubled Windows machine and go to Logical disk manager to format it, I discover that the partition appears as a GPT protective partition and you cannot do anything to it.

Cleaning a GUID Partition table

Long story short: having been part of a zpools, that disk was labeled with an EFI label. EFI adds support for GUID partition tables, and that (partially) explained the behavior of the Logical disk manager. I didn't even imagine that Windows XP would know about EFI of GUID partitions tables and I understand why it didn't let me touch that disk. What I didn't know was how to clean the disk to be able to format it with NTFS. Fortunately, Google always helps who knows what to search, and I discovered about the existence of a command line utility called diskpart. With diskpart it was easy to clean the disk and format it again. Instructions are straightforward:
  • open a command prompt
  • launch diskpart
  • list disks with the list disk command
  • select the disk you want to clean with the select disk command
  • clean the disk with the clean command
That's it.


John said...

Oh man, thank you so much! I had an external HDD (Seagate FreeAgent) that was formatted on a Mac but I wanted to switch it back to my old PC. I plugged it in and, unsurprisingly, couldn't get a drive letter. All I wanted to do was format it, but I couldn't even do that through Disk Management until I followed these command prompt steps. Thanks again!

Grey said...

You're welcome. John.

I wrote this because I supposed it would "hurt" many people, and so it seems to be.


zlenka said...