I recently decided to attempt to turn my internal HDD into an external one to save me repeatedly opening my laptop case to remove it whenever I wanted to use other operating systems. I assumed this would be a simple matter of converting it to external then plugging it into the USB 3.0 slot, however despite recognizing the drive and WBM on it it simply won't boot properly and displays a message about problems and needing to restart. I've tried fiddling around with UEFI and other BIOS settings as well as performing startup diagnostics but no luck.

I figured you would be able to run Windows 8.1 directly from an internal HDD converted to external HDD with no internal HDD, is this not the case or am I just missing some settings I need to change in BIOS?

Sorry if I'm not being clear enough or haven't provided enough information (just let me know and I'll inform as best I can), any help would be massively appreciated.

Does the external HDD case have a dedicated power cable? or is it using the power from the USB 3 port? If it's the latter then it may simply be that the drive isn't booting fast enough for the computer to be able to use the OS on it.

Windows really hates booting from external drives. You can Google around for some instructions and you may run into further issues related to your motherboard too.

It does not, it's 2.5" so I was under the impression the power from the USB port would be sufficient from what I read. Would an enclosure with its own power supply solve this problem?

Google was my first stop, but I had little luck there as most people seemed to be using externals in combination with internal drives.

Take a look here: http://pcsupport.about.com/od/tipstricks/ht/bootusbflash.htm

Power may be a problem depending on the drive, but then you won't be able to recognize the drive whereas your op describes having accessed the windows boot manager.

Google around for installation instructions on Windows external drives or registry edit. Windows doesn't like booting over USB.

Linux on the other hand has no issues with it and just works^tm

I'll quote you on that next time I try to set up Gentoo.

Windows is already installed on the HDD. I figured it was just a matter of removing the drive, converting it to external and plugging it in and it would work as normal and pick up where it left off. I'll try googling for getting it to boot through USB.

Yeah, I boot two distros from disc usually and it's super simple (just a matter of changing some BIOS settings), figured it would be as simple for Windows, I should have known better

I said nothing about setting up . Also gentoo!= linux. Linux is definitely far better at running on any given heap of hardware (the kernel has much wider hardware support) and many distros thrive solely on being bootable directly into a usable environment. And the gui installers on linux are still leaps and bounds easier to complete than Windows' installer

So, uh, I'm probably not going to be able to run this HDD as external unless I format it first (and then various other steps), it seems. I would be as well just starting with a fresh HDD then, really, and even then it will probably [censored] up somehow. Alright, screw it, I'll just keep opening it up, not a massive hassle anyway, really.

Why are you swapping hdds anyway? Just use partitions, VMs,and if need be, PXE

I'm not swapping HDDs, the distros I use boot from disc and run entirely in RAM, so when I use them I like to disconnect all HDDs, little paranoid in that regard (especially since the HDD in question contains Windows 8.1 on it). I hate Windows really (even more so now, hehe) and only use it for gaming and things the distros can't handle (which is quite a bit considering they're RAM-based and super lightweight). If I'm ever just browsing the Internet or doing other things I'll use the distros. Which, by the way, are lightning fast compared to Windows since they run entirely in RAM.

Problem is the laptop I bought seemed to be designed to keep people from opening it up, in fact I'm surprised they didn't just weld the plastic case shut, hehe. So it's a little bit of a hassle to keep removing and putting the HDD in every now and then, not as easy as just sliding open a case. I'm more worried about wearing down screws for the case near the disc drive, but I can always get replacements I suppose.

Thanks for the help everyone, by the way, in case I forgot to mention that already.