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I have a Drobo 4-bay (2nd generation) which was originally filled with 1 TB drives. Over time, I've replaced the 1 TB drives with 2 TB drives, and now I want to re-use one of the 1 TB drives for other purposes. I know I'll have to re-format it, that's no problem. I have an enclosure for it, and I installed it correctly in the enclosure, but when I plugged it in, it won't mount.

Is this a typical problem with a drive that's been in a Drobo? Is there something I should do to get it to mount so that I can reformat it?

Update: Sorry, I should have specified the system particulars. I'm running OS X 10.6 Snow Leopard, up-to-date. The Drobo is running firmware version 1.3.7. The drive doesn't show up in the Finder nor in Disk Utility.

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Which Operating System are you using? (If using MS-Windows, please include Service Pack level details as well.) – Randolf Richardson Jun 18 '11 at 23:37
Do all of the 1 TB drives experience this? I would think that the drive would show up in Disk as long as it and the external enclosure was working correctly. Do you have another enclosure to test with? – Graham Jun 19 '11 at 4:05
Since you already know you need to format it before use, have you actually done this before expecting a volume to show up in Finder? You didn't mention. – Daniel Beck Jun 19 '11 at 5:42
I suspect that your problem may be that the enclosure you're trying to use can't handle drives that are 1 TB in size -- I've had this problem with older enclosures only being able to support drives that are less than 1 TB, and then later on I experienced the same problem again with drives greater than 1.5 TB. If you can try a newer model of enclosure, you may find that this resolves your problem. Alternatively, opening up your chassis and plugging the drive in directly as a secondary would help to prove this also. – Randolf Richardson Jun 19 '11 at 15:09

If Windows, did you look for it in Disk Management?

When pulling a drive from a proprietary disk format device, I would just wipe it with DBAN first - then it should be a "raw" disk that should be usable in anything, once you partition and format it.

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Or just use fdisk to write a new mbr and partition table. – Keith Jun 19 '11 at 3:39
I assume you mean in Linux? Fdisk doesn't exist in Windows these days - it was a DOS/Win9x tool and those haven't been around for 10 years. – Multiverse IT Jun 20 '11 at 16:45
The OP is running OS X, not Windows. But yes, it's been over ten years since I've used Windows for anything. :-D – Keith Jun 20 '11 at 17:19

The drive will not mount, as the drive doesn't have a file system OS X understands, so you need to Erase it. If it doesn't show up in Disk Utility, this indicates the hardware is not seen, you will need to correct this before you can complete the steps below:

  1. Open up Disk Utility (Finder > Applications > Utilities > Disk Utility)
  2. Select the correct drive.
  3. Click the Erase tab
  4. If you want the drive to be used on both the PC and Mac, you will need to format the drive as MSDOS format, otherwise use the default.

This will make the drive mountable again and also format it.

If you can't get the Mac to see it try this on Windows machine:

To "clean" the drive in Windows:

You can also use DISKPART to clean the drive.

  1. Open up command line.
  2. Type diskpart and hit Enter.
  3. Type list disk, you will see a list of all available disks on the system.
  4. Once you think you have the correct disk, open up Disk Management and verify it is the correct disk number. Disk management is the just the GUI version of diskpart, but it doesn't include the clean ability.
  5. Now type select disk X, replace X with with the correct disk number.
  6. Type clean and hit enter.

The drive will now be uninitialized, you can now initialize it and format it as you wish in Disk Management GUI.

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Thanks for the suggestions! In fact, the drive doesn't show up in Disk Utility, so as you suggest, I'll need to correct that---do you have any thoughts as to how I can do this? – Alex Basson Jun 19 '11 at 14:16
Try the drive with another enclosure or get a IDE/SATA to USB adapter. – Sean Jun 23 '11 at 18:44

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