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Help Converting From MBR to GPT

Brian Crowe
Brian Crowe asked
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Last Modified: 2015-10-06
I have a very active data disk that was unfortunately formatted as MBR in Windows 2008 R2.  It is quickly approaching the 2TB limit.  I’m considering something like AOMIE Partition Assistant or Terabyte BootIt Bare Metal.  Both seem to convert from MBR to GPT but I am worried about doing it on a volume that is actively being used and so close to the 2TB limit.  The volume is actually at the 2TB limit and only has about 200GB of free space.  Does the amount of free space make a difference?  Any real world experience converting MBR to GPT with either of these tools?
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Lee W, MVPTechnology and Business Process Advisor
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Commented:
No experience with these tools and frankly, wouldn't use them.

If the data is important, install new drives and set them up correctly first.  Then move the data.  You could use Robocopy to Mirror the data from the old drives to the new and then have a 2 minute window of inaccessibility (probably faster than the time it takes to use some third party program) while you swap drive letters.

(I assume these drives are already larger than 2TB?  Why wouldn't you have set them up as GPT in the first place - if they were larger than 2TB...).

In either case, MAKE FULL BACKUPS FIRST!  You're messing with partitions - you never know when something won't work right because of some odd, seemingly minor (never even thought of) difference in your computer.
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Commented:
If the disk is only 2TB in size, converting it to GPT won't help anyway. It would only be of help if the disk was larger than 2TB in the first place.

Also, if this is the partition you are booting your OS from, converting it to GPT will get you an unbootable OS. For an m$ OS to be able to boot from a GPT disk, you fist need an UEFI BIOS, and it has to be installed using EFI mode from the install media, and the destination must also be a GPT disk.

So basically we need more info on your environment to rule out those possibilities...

If changing to GPT really applies, then I would backup, delete any partitions on the disk so you can convert it to GPT directly from within diskmanagement, create a new partition and restore your backup.
Brian CroweDatabase Engineer
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Commented:
I posted this question on behalf of our system admin so bear with me as I try to answer as best I can (I am just a lowly DBA).  

The disk in question is not a boot disk.  It is just a very active repository for customer-related files that has grown faster than expected.

The machine is virtual and the disk itself is a logical SAN drive.

What additional information would be helpful?
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Commented:
Then I would just create a new virtual disk to the VM and make that GPT. Then restore the data from the backup of the old disk to the new virtual disk. This is much simpler and particularly safer than converting a disk.
Brian CroweDatabase Engineer
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Commented:
We are aware of the brute force alternative and that may be what we need to go with.  We are just exploring other options.  Although I am a relative noob at this stuff my colleague is not.

The question is whether these or any other conversion tools actually work for converting an existing MBR drive to GPT while that drive is active in the real world.  

Will we lose connectivity to the drive during the conversion?  How long will it take on a drive this size?  Has anyone actually tried it with or without success?
Lee W, MVPTechnology and Business Process Advisor
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Most Valuable Expert 2013

Commented:
You haven't mentioned what the size of the virtual disk is and I'm not seeing any indication of what virtualization platform you're using.  Frankly, you're leaving a lot of details out.

As has been stated if the disk is a 2TB virtual drive already, the virtualization technology may limit it's size.  Hyper-V VHD size limits are 2TB.  Unless you convert it to VHDX - but that has to be done offline.  How fast depends on what the platform hardware is - SSD?  15K SAS in a RAID 10?  RAID 5?  RAID 6?  How many drives?  What's the RAID controller - at the end of the day, it's foolish for us to give or you to accept any time estimate because there are so many variables.

You're asking about software to do something that most professionals simply don't use.  If you want to see how it will work/react, the best thing to do is run it in a test environment.  You clearly have a virtualization plaftform - it should take less than an hour to setup a test system and explore how it works/what happens.  You can use FSUTIL to create dummy files INSTANTLY of any size you want so you can test with x% of the disk full.  

I know you want a simple, reliable answer... problem is, from what I know of most professionals and what you're asking, this is high risk - and high risk things are avoided by professionals when other, lower risk and better alternatives exist - as I think they do in this case.

Good luck.
IT Product Manager
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Commented:
This one is on us!
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