Helping people with computers... one answer at a time.

C: is the most common designation for the hard drive from which you boot and which contains Windows. It doesn't have to be C: but it's safer if it is.

I just added new hard drive to a tower. I moved the old drive with windows on it to slave and installed XP pro on the new drive. Everything looks dandy but the new boot drive is labeled F: and the old drive C:. Is this going to cause troubles for me later on, and can I or should I bother to change it? You did a version of this before, but I wasn't sure if C was sacred, or if new software that needs to monkey with Windows can figure things out like that by itself.

Well, there's actually two answers to this: yes "C:" is sacred, and no, it's not.

I know, that wasn't very helpful.

But I do have some practical advice and some bad news about changing what you have.

"C:" is not supposed to be special any more, though once upon a time it was. When hard disks were first added to personal computers A: and B: were assigned to the floppy drives, and C: was the hard drive. In fact, C: could be counted on to be the hard drive, the boot drive, and the drive containing the operating system.

But it's been quite a while since that was a valid assumption. As you've seen today other drive letters can now end up as the boot/Windows drive, and things pretty much work.

The risk, of course, is some software making an incorrect assumption that C: is the Windows drive. Fortunately those situations seem to be infrequent, and getting more so.

Advice #1: If it ain't broke... don't fix it. As we'll see in a moment fixing this gets painful. Things should work just fine with the setup you have. You may have a mental hurdle to overcome in remembering that C: isn't the boot drive (I know I would), but unless you run into a problem that can be traced to the drive assignments, I wouldn't change a thing.

"In my opinion the only safe way to change the letter assigned to your Windows drive is to reinstall Windows."

Advice #2: Pseudo-standards are good. When you build out a new machine I do recommend, if you can, making sure that you end up with a system that has its boot drive as C:. As I said, it shouldn't be necessary, and it's not really a "standard", per se. But it is one of those things that can make life a little easier in case you do run into situations where some software makes a bad assumption. Given that 99% of the machines out there are configured this way, it's just a safer thing to do.

Sad Reality #1: Change is painful. Changing drive letters is easy - as long as you're not talking about the drive that holds Windows. In my opinion the only safe way to change the letter assigned to your Windows drive is to reinstall Windows.

Here's the problem: installed software, including Windows itself, will have been configured to whatever the Windows drive was when they were installed. Using your example, by that I mean that in various configuration files including but certainly not limited to the registry will be references to F:. Lots and lots of references to F:. Unfortunately changing the drive's letter assignment won't go through and find and change all those references to the old configuration.

If you do successfully change F: to C:, then a lot of things are likely to stop working. In fact, I'd actually be shocked if the system even booted.

Now, one could argue that you could just scan the registry for all references to F:, change them to C:, and be on your way. It wouldn't surprise me if there were tools out there that do this for you. And it might even work in many cases. The problem is that frequently the registry is not the only place information is stored. Let's say that some random application you use is configured not to use the registry and stores its own configuration data itself, in a proprietary format. You may be able to fix it when you run the application, but you may not. And it's an annoyance in any case.

That's why I say that the safest way to reassign drive letters involving the Windows drive is to reinstall. That way everything is configured properly from the start.

I'd love to hear some reader's experiences on this topic. As you can tell, reassigning the Windows drive letter is something I actively avoid.

Article C3270 - January 17, 2008 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Ethan R.
January 17, 2008 12:39 PM

I wasn't paying attention when I was building a new system for my wife and myself about six months ago. There were flash card readers installed on the internal USB ports that took all of the lower drive assignments, so the single hard drive ended up as H:. I went with the "If it ain't broke" theory and left it as is. I've run into problems a handful of times. Most of my problems have related to programs that won't install on any drive other than C:. Again, its only been one or two shareware type programs. Once I had an auto-executing unzip package that would only unzip to C:. I stuck a flash card in the appropriate slot and it unzipped without any problem. So far I'm glad I didn't go through the hassle of reinstalling Windows, which I'm sure is the only way to change things.

Duncan Bachen
January 17, 2008 1:00 PM

The problem comes into play with the doing a HD upgrade as many folks want to do now in the digital storage age. They simply outgrow their old HDs. When you use the HD cloning software that comes with the drive, it clones it, but the system and Windows XP will likely make it be a new drive letter such as F:, especially if the drives are setup as Cable Select

Reinstalling is one option, assuming the drives are in the right 'order' with the CS. The other is to actually go into the Windows XP and change the drive letter of the mounted system drive. That way your new "f" drive is really the "c" and all the installed programs start working again. This is a registry hack only, because you normally cannot change the drive letter of the active system.

Here's a MSKB article which gives you the correct key information:

January 17, 2008 1:03 PM

I had this happen to me once, I wrongly assumed it would assign C: to the “main” hard drive/partition, regardless of what the other Windows installation was doing.

At first I thought nothing of it, but it got frustrating after a while as programs were assuming that Windows was set up on C:. I ended up just reinstalling Windows.

Ah the joys of Windows and it’s ridiculous file system layout.

Greg Bulmash
January 17, 2008 1:50 PM

One of the problems is that some older programs or packages may need to be reinstalled because they rely on an absolute path ("C:\program files\...") to locate libraries or data files that they need. If those files are moved to drive F:, the programs will give you a "can't find..." error and refuse to boot up.

January 18, 2008 8:29 PM

If this happened to me, I would try Registry Registar's registry editors to do a search and replace. I would search for X: and replace with C: . This should work, but haven't tried it.

They have a free version too. Of course I would back up the reg with ERUNT first.

Fortaleza Computer Doctor
January 19, 2008 6:15 AM

To eliminate the situation completely.. Use only one drive connected to install your Windows, then put the other drive online after everything is working to your satisfaction. It will automatically become D or another letter depending on how many CD, DVD or USB drives are connected. As a bonus, the old drive may still be bootable and you can even change the boot drive settings in the BIOS and use the other original system.. in other words, the C letter will always be the boot drive!

Another tip for changing the drive letter: Search for Change of Address (COA2) from Ziff Davis, it appeared free in PC Magazine back in 2001. This ulility will change all incorrect drive letters in the registry with one click! This utility was made for the very purpose your article talks about.

Gregory Harris
January 19, 2008 2:38 PM

So far after all these years I have never had that problem..........why?? because I am chicken and always follow what my father always taught me.....if it aint broke do not try to fix it. Yes, after reading the above article I would always follow the if it is not broken do not try to fix it and with me drive C is always the hard drive as in school that is what they taught us some forty years ago.

Richard Broberg
January 19, 2008 8:35 PM

I agree completely with the Fortaleza Computer Doctor. I always have just the master drive installed when I install Windows. That forces Windows to install on the master drive which will have at least a C: drive on it. I always devote the entire master to the C: drive.

After Windows is installed, then I add the slave drive, which is where I keep my data.

One time I installed Windows when I had both the master and slave drives installed and Windows installed on the slave drive. I don’t know what it or I was thinking.

I never change drive letters. Windows is to unpredictable as it is.

January 20, 2008 4:27 AM

When I first installed XP I upgraded from 2K Pro. Rather than reformat my system (I may not have liked it after all), I threw in an extra drive and installed to it instead.

XP got installed the letter I. 2K Pro had a letter of C. Eventually a third drive got placed in the system and assigned J.

The only problem I ever ran into, is that for some reason whenever I'd click on the shortcuts in a Save as dialog (on the Places Toolbar) it would default to my C drive. It took me a while to find shortcuts that had been saved on my windows 2k desktop.

January 25, 2008 4:29 AM

Let me drop this comment into your minds. C is really sacred just as A and E are also sacred. By default C was made as the boot drive. Its permanantly wired in the CMOS chip and by default the A, C and E are there by default, thus are sacred.

December 23, 2008 10:58 AM

Have HP Windows Xp SP 3, all is weorking fine, My Local drive, windows changed the Letter to "L", I still have drive C, and also a drive I, which it gave me. for some reason. anyhow, I have an external HD, "My Book" 500 gigs, is installed, through its normal installation program, but it does not show up in "My Computer" as a HD, and I always haver to do a search for it. I thought I assigned it when it first installed, but in the systems area, it does not show up anywhere, it did af first, but is not there, now, My question is how to get Windows to recognize my external HD, which is named "M",.. Thank you so much.

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