I have a computer running windows XP but our family has a computer running Vista. Can I use an external hard drive to make a mirror image of the Vista computer's hard drive and use it to boot Vista on the XP computer if I have it be the master Hard Drive.
Thanks

Hello.

I don't mean to put a wet blanket on your idea, but most would say no...can't boot from an external hard drive. However...hope runs eternal...check the guides forum and you will see there are some who say, "yes you can." Unfortunately I can't remember the different authors names, but one specifically said he couldn't get a specific hard drive to run only on the original computer, as the original topic of one guide, but he tried a different means and got it to boot....on most computers, not just the original.
I'm trying to do something very similar to your idea, unfortunately I'm not well versed in DOS and boot systems so it is slow going for me.

Ok. Thanks. I'm just trying to cut my costs.

I understand.....I have two 500GB self powered USB external drives that I want to use. One as an independent operating system for priority work using XP Pro SP3, and one as a duplicate drive of my original C drive for an insurance policy of a mechanical failure of the SATA drive in a Dell. All my computers are XP Pro with their OS disks except for a Vista laptop (no disk, since it was a factory install).

I added the two names but I see they didn't stick. The guide is by nobardin (How to install and boot Windows on an external (USB) hard drive) with a response from stealthmode with an alternative means.
Before you try nobardin's work, read the whole thread for later tweaks. I'm out of CD disks now, so I can't try it again until my supply arrives. The first try didn't work for booting, but it was recognized as a drive....missed it by ---that much. LOL.

PS>>>>don't let all the techno-speak scare you out of the "process." Downloading and installing the few programs clears a lot of "aw-craps." The step by step hand holding is excellent for novices like me with DOS even if you don't understand all of the interrelationships that are going on with the process.

Thanks. If that doesn't work I will also try to use a program that exactly mirrors an existing hard drive and use that to boot. The thing that worries me though is that my computer was made before 2000 and though it has USB ports, I don't know if it can boot from one.

That's the first thing to check. You should have a PF key or keys to get you to the bios screen upon booting your computer (turning it on for operation). For Dells it is PF 12, others may be PF 8 or PF 2.
Once you get there you should see the choice of booting, generally floppy disk, CD drive, and then internal drives. One of the last choices is USB. If it is not listed you are done.
You can't add it to the list.

This depends mostly on what you are trying to use the HDD for if it's for transferring system type files eg. Jpeg, jpeg, mp3, mp4 etc. etc. then try a free O/S http://www.ubuntu.com/getubuntu/download
There is quite a long list of compatible file transfer types included in the 'downloadable' you'll need to be able to create / burn an 'Iso' boot disc https://help.ubuntu.com/community/BurningIsoHowto
I haven't tried what I'm suggesting but I know that the only other thing that you'd need to do is to format the HDD as FAT32 instead of NFTS due to your (?possible?) USB 1.5 ports.

Ucould2,
He wants to boot Vista to an XP machine from an external hard drive.
If he doesn't have a USB boot option, he could boot from a CD if all he wanted was the operating system. I believe he wants to clone the Vista machine so he has all available data and programs to run on his XP machine from the USB drive, maintaining the XP on the system drive when he isn't using the Vista drive. This will not happen if he has no USB boot option.
He could also partition the system drive to run two operating systems (XP & Vista), but that doesn't afford him the use of his USB drive as a boot source, only as a data storage source.

Well, I just want to get vista, I don't really care about the files on my vista machine. Could I do an iso disk, boot from it, and put that on my internal hard drive? I am going to get an external hdd anyway since my existing one is 40 gb.
I looked at that Ubuntu thing and it looks pretty cool. Can I switch between booting that and booting windows?

system specs? noticed you have two computers? one computer with XP O/S and another computer Vista O/S? which system to you have a boot disc for? do you have a boot disc?
as was mentioned before @pepsi1 you can partition your HDD and run separate O/S systems for different uses however with only 40Gb you may fill it quickly. This is big enough to try it and if you run out of space then look at the external drive option.
On the external drive as the boot drive I've read threads where SDHC flash cards are the boot system memory. If you have the drive and a card you could try this with the Ubuntu at the cost of the Iso CD

Specs: Vista PC: HP pavilion a6500f, 4 GB RAM, 500 GB HDD, Vista home premium factory installed
XP: eMachines T2642, 256MB RAM, 40 GB HDD, (no boot disk)
I don't have a boot disk for either of the pc's.
The XP computer is a very old computer (over 10 years old) so it does not have any type of modern/fancy ports other than USB 2.0, ethernet, and phone jacks.

So could I partition my 40 GB HDD to have Ubuntu and XP, use an ISO disk to install Ubuntu and then boot from the HDD in the future?

HP is the better of the two units but you already know that. Without any boot discs doing any thing could be non-beneficial (you could fragment the file management and render the O/S useless). However being that the HP is the newest I might assume that you may be able to take it back to the place of purchase if 'something unthinkable happens', more so than the 10 year OLD system. Lets go hypothetical for a moment
~remove the older 40Gb memory and connect it to the newer system create a mirror XP profile on the Vista machine.
~purchase new HDD fit it into the emachine, partition new drive so that it could transfer files etc from the 40Gb HDD with Ubuntu in own it's partition so the emachine would still operate as a computer (standalone) extending it's lifespan
~the older 40Gb HDD should still be able to boot the emachine through the "VT6421 SATA+IDE Extend Card" using the "HDD Docking Station"
~the new memory would need a "Bidirectional IDE SATA Conversion Adapter" to run in the emachine
~the anti-static wrist strap because I wouldn't want to hear that you've fried something
This configuration requires some knowledge of the emachine as you will be fitting the "VT6421 SATA+IDE Extend Card" into the PCI slot (does it have one? upload photo of MoBo?) PCI because you'll be able to use it later in the HP! and the "Bidirectional IDE SATA Conversion Adapter" to the NEW memory, these all require power cords (are there any spare in there?or purchase?)
Will the Vista be able to operate the XP files? yes
Will the HP allow you to partition the drive so that there is another O/S on the 500Gb drive?yes
If you somehow managed to transfer the start-up files from the 40Gb drive into the partition you've created on the 500Gb drive would Vista allow you to run the copyrighted material?no
If you going to do anything from this list before you do. Update Windows Vista to Windows Vista Service Pack 2 (SP2) so that you'll have a restore point if that 'something unthinkable happens' you will be able to undo
Ok so you don't know how to do anything here? no probs try this; Ring the place of purchase and ask if you can take the HP back to upgrade your machine to the Service Pack 2 (SP2) and book it in. Arrive there with both computers purchase the anti-static wrist strap and the power cables you'll need for the card and converter ask them to show you where all the parts go in the emachine and that will be your first lesson......the worlds your oyster the only way is up!