Eric Stewart: Running Off At The Mouth

Disk Imaging and Changing Your System Drive Letter

by Eric Stewart on Oct.17, 2009, under Computers, Technology

First, some background, because some folks might find it interesting as to how you get to where you are:

So I had this 500 GB SATA drive laying around.  From where?  Well, for a while there I was a Bright House customer and had the drive in an external bay to expand my DVR’s capacity.  It became unused when I moved to Verizon’s FiOS service.

Somewhat concerned about software (and hardware) compatibility as I planned to go with the 64 bit version of Windows 7 when I did the actual upgrade, I decided to copy an image of my 250 GB RAID 1 array to another drive, ensure I could boot to it and use it normally, and the blow away the WinXP on the array and install Windows 7 on it, and then get the system to dual boot Win7 and WinXP.  Eventually, when I resolve all of the issues (if possible) I needed to with Win7, the WinXP would be either archived (backed up/image archived) and the drive freed up for other use.

So the 500GB drive fit the bill nicely.  Did have some issues with getting it into the system; if it weren’t for the fact that the two 250GB drives were purchased right about the time SATA drives came out, I would have had to do something about the power supply.  Thankfully the 250s could use the older Molex 8981 Series/AMP Commercial MATE-N-LOK connectors, as I had a limited number of  SATA connectors (only two, and I would have needed three).

Now, when I talk of disk imaging, there are a few products out there that do what I’m talking about.  I was using Symantec Ghost borrowed from work.  I just did a straight disk-to-disk copy, but if you’re going to do anything like what I’m suggesting I would advise you to do as I say and not as I did, and first take a copy of your disk to an image somewhere that you can use as a true backup.

So, I installed the disk, booted up Windows XP (this is before imaging), and did some perfunctory checks on the drive to see if there would be any problems with the hardware (in other words, I ran a chkdsk /r on it).  An hour or so later (the first four stages of the five stage check going instantly on the essentially empty drive; that fifth stage being the “free space verification” taking almost all of the time), everything came back hunky-dory, and I started the imaging.

Another hour and 15 minutes or so later (yes, there’s a lot of stuff on that WinXP install), I had the image copied.  I set up the system to dual boot, and this is where things got a little weird.

Something to do that might make troubleshooting (and resolving) this situation easier, if you’re not as lazy as I am and trust that you can undo everything you did, that you might want to do is go through the process of totally disabling the particular drive/installation of the system you don’t want to boot into while you work on the other.  But I live dangerously, and this is not what I did.

So, let’s face it: The drives are supposed to be near exact copies of one another.  Telling the difference between them required dropping a file into the Desktop folder on each of them (you can do this with only one of the drives booted, if both are mounted to the install in question) indicating that I was looking a different desktop than the other.

Complicating the matter further was that, even though I had changed the drive labeling on each drive, “C:” always showed up as the 250GB RAID, and “F:” always showed up as the 500GB drive.  Even when I was supposedly booted to it.  Further confusing the issue was that while some things, when booting to the 500GB drive, would point at F:, a lot of the program files would point at C:.  As you might guess, things, when booted to the 500GB drive, were less than stable.

This is where we learn about some internal Windows stuff.

So, we’re talking exact copies.  The 500GB drive would come up enough to browse files, but we’d see all sorts of svchost.exe errors and eventually the system would freeze (particularly if you close out the svchost.exe error messages – PROTIP: Leave the error messages open until you’re ready to reboot).  As you might have guessed, Windows doesn’t assume the boot drive is C:.  It has, internally, some description of the drives on your system (IE, where in the basic hardware layer they exist, such as “There’s this drive the BIOS is telling me is drive 0 and I will name that C:”).

Now, if you’ve played with Windows enough you know that you can right click My Computer, go to “Manage”, and one of the options you have is “Disk Management”.  In that you’re allowed to change the drive letters of your drives.

Most of them, anyway … certainly not your boot drive!

There is a trick here, and you should read all of the Microsoft Knowledge Base article about Changing the System Drive Letter before attempting this.  Here’s the tidbits you’re interested in and what I ran into that was different (and worried me a little bit):

  • The registry key in question is at HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
  • All you’re really going to do is “Rename” some keys so that \DosDevices\C: isn’t at C:, but something currently unused, and whatever you need to be \DosDevices\C:, rename it so.  You will of course leave the key values alone.
  • The instructions involve using RegEdt32.exe to change permissions on the key so that an admin can edit it, but using RegEdit.exe to do the actual key renaming.  Here’s where it got really weird for me:
    1. RegEdt32.exe looked exactly like RegEdit.exe on my system, and I know it’s not supposed to.  I haven’t used RegEdt32.exe in a long while but I know it looks a lot different.  I do not know at this time why things are this way, and not sure I want to go mucking about to figure it out.
    2. Full permissions on the keys in question were already available to me.

But, a quick couple of renames and a reboot later and the 500GB drive installation works just like the 250GB RAID installation did (though I haven’t gone back and tested the 250GB RAID installation since I did the registry editing … unless you read otherwise later, assume it’s fine … it may not matter since the 250GB system is going to get rebuilt – probably tomorrow).

So, that’s pretty much it for now.  This was quick-published as I have errands to run today … if I make substantial changes, I’ll tweet that I did so. 🙂

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