Booting off an old HD

I recently ordered one of those toaster-style HD docks for my laptop so I could use my old desktop HD for storage.  However, I just read something about booting from external drives and was wondering if this means it's possible to run XP (which is installed on my old HD) instead of Vista (which is on my laptop) on my laptop.  Is this how it works or did I misunderstand?

If it's connected via USB, I don't think you can boot an OS off it... I could be wrong, though.

CORRECTION:
well, maybe you can, but looks like there's a process... this site talks about using a USB flash drive, so I'm not sure if it works with a USB to IDE/SATA drive:
http://articles.techrepublic.com.com/5100-22_11-5928902.html

I was in my BIOS and it gave an option to boot off a usb device. I guess I should clarify what I really want to know- does "booting" from a drive mean that I'm running whatever operating system is on it?

Well, if the OS was previously properly installed on the drive, you may be able to boot it with a "boot from USB" option. Give it a try, let me know what happens

electricmonk9 said:

I was in my BIOS and it gave an option to boot off a usb device.  I guess I should clarify what I really want to know- does "booting" from a drive mean that I'm running whatever operating system is on it?

Click to expand...

You sure it was not the option to write a new BIOS firmware via a USB drive?

I don't think so.  I entered the menu on startup and it asked me to pick "the desired boot device".  The options looked like

Internal HDD
USB Storage Device
CD/DVD/CD-RW Drive
Cardbus MIC (Maybe NIC?)
Onboard MIC

then underneath:
BIOS Setup
Diagnostics

The default option is "Internal HDD".

:edit: @rkcstr- I don't have the toaster yet.  It's being shipped to my house where it will hang out with my old HD without me until I return home from college in a month.

Yes, if your bios lets you boot to USB devices, and there's an install of XP on a harddrive that you can plug in via USB, then yes, in theory you should be able to boot it.

The problem though, is that windows is set up to use your old hardware... It's assuming you have X brand of chipset, and X brand of IDE controller, and sometimes (quite often) you'll end up booting off the external drive only to find yourself with a blue screen of death saying STOP 0x0000007b.

I've found a workaround to patch the registry to accept the new hardware, but I've had mixed luck with it. You can have a look here, though again, I've only had that work a couple times. You could also try deleting the HKLM\System\CurrentControlSet\enum key from the registry, I've had that work a few times as well, though this sort of thing is well beyond the experience of most users, and like I said, it really only has a 50% chance of working.

Wow, that sounds complicated but I'll definitely give it a try. Thanks Psuedo!