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On Windows 7 x64 Professional I am struggling to convert a basic disk to a dynamic one. Under Disk Management in the MMC the conversion is supposed to be initiated automatically, but it doesn't.

My guess: because of using third-party partitioning tools there isn't enough space in front and after the partitions (system-reserved/boot + system volume) to store the required meta-data. When demoting a dynamic disk to a basic disk manually, I noticed that some space seems to be required before and after the partitions.

What are the exact alignment requirements that allow the on-board tools in Windows to do the conversion?

I have a Windows 7 x64 Pro installation that has been transplanted onto a new hard drive. All works fine and smoothly so far. However, since the old disks were starting to fail, I removed them, demoted the existing RAID1 manually (by converting the volumes to "basic") and then transplanted it onto the new disk.

This happened by means of Acronis TrueImage Home (2011, in case this matters).

Now, after restoration of normal system functionality it turns out that when I try to create a RAID1 (mirror) via Disk Management in the MMC, it tells me there's not enough space to convert the basic disk to a dynamic disk, which is a prerequisite to build the software RAID1.

As far as I understand the relation here from demoting the (one) dynamic disk (of the previous RAID1), there is a certain amount of "free" space required in front of the system partition and/or alignment requirements.

I need to know those requirements and preferably a tool that allows me lossless application of those requirements so that the onboard Disk Management tool of Windows is able to build the RAID1.

The boot volume/partition (System Reserved) is 100M and the rest of the disk makes up the system partition (C:).

Here's the screenshot, I have no idea how it would help in addition to the description, but anyway ...

Screenshot of the reported error

This is a VM that I used to reproduce the condition I am seeing on the actual machine.

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This maybe be obvious, but make sure you have proper backup of your data before you are going to do these kind of partition acrobatics. :) – Mxx Oct 15 '12 at 7:05
A screenshot of the diskmgmt.msc would really help – Viral Jain Oct 15 '12 at 7:11
up vote 2 down vote accepted

I believe I have heard it said that at least 1 MB of unallocated space is required at the end of the disk to create the database for the dynamic disk.

When Windows creates the partition it does this automatically, and this unallocated space is not shown in Disk Management with Windows 7. Win2K and early versions of XP will show this space, typically 7-8 MB. I think MS decided to hide it so people would stop asking why that space couldn't be included in a partition.

If you created or resized the partitions using a 3rd party tool, this space may have been allocated, which will prevent converting to Dynamic.

My advice is therefore to shrink the partition so as to leave at least 100 MB of free space at the end of the disk.

If this doesn't work, one of the usual workarounds is to delete all partitions, reinstall a vanilla Windows 7 without activation and then restore the image, since the newly created partition will be correctly allocated. Partition sizes are less important, since (if I am not wrong) Acronis can restore an image to a different-sized disk.

The MS article Dynamic Disks states :

Dynamic disks offer greater flexibility for volume management because they use a database to track information about dynamic volumes on the disk and about other dynamic disks in the computer. Because each dynamic disk in a computer stores a replica of the dynamic disk database, for example, a corrupted dynamic disk database can repair one dynamic disk by using the database on another dynamic disk. The location of the database is determined by the partition style of the disk. On MBR partitions, the database is contained in the last 1 megabyte (MB) of the disk. On GPT partitions, the database is contained in a 1-MB reserved (hidden) partition.

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Perhaps to be on the safe side, 100 MB in front and 100 MB in the back. Once disk is converted, if that disk space is important, grow it to full size. – Mxx Oct 15 '12 at 7:05
@harrymc: I wrote up an answer detailing what worked in the end. Even though most of your answer seems to reiterate what I wrote, I think the 1 MiB piece of information is good enough for now. I was anyway experimenting in my VM to find out and would have preferred any kind of authoritative answer, but I guess this one will have to so. Alignment seems to be relevant only for the start of the partition(s) and the "free" space, it seems, also has to be aligned somehow. I'll probably award the bounty to you if I get no authoritative answer ... – 0xC0000022L Oct 17 '12 at 21:35
I added some info from a MS source according to which 1 MiB is enough, also detailing its format for MBR & GPT. – harrymc Oct 18 '12 at 7:26

It turns out there are two requirements:

  1. 1 MiB alignment (mentioned by harrymc). This requirement exists since Windows Vista.
  2. somewhere between 2 and 4 MiB of free space after the last partition for the meta-data.

Space between the partitions doesn't seem to be required, nor is space in front of the System Reserved partition required.

Once I had that, the disks could be converted to dynamic disks and could become members of a software RAID1.

I'd still prefer an authoritative link/source concerning this, nevertheless.

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Consider using Parted Magic to losslessly* resize/move your partition(s). Parted Magic is a LiveCD/USB Linux distribution with a nice simple to use visual tool to make changes to your partition. You can use it to change your partition so that there's sufficient unallocated space in front and behind of it.

*make sure you have backups of your data.

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the other tools that I use do exactly that, but that doesn't guarantee alignment. Although, of course, some tools have options to set a particular alignment. – 0xC0000022L Oct 15 '12 at 10:06
It was obvious that some program was going to be needed to do this and I wasn't asking for a suggestion what tool to use but rather a very specific information how to apply the tools I had. – 0xC0000022L Oct 17 '12 at 21:31

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