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Back in April, when I first got my 2011 Macbook Pro, I ran into this same exact problem and wrote the the following in another post. This worked for me and its a lot of steps and swapping of drives a couple of times, but it works like a charm. Hope this will help:

Here's the deal. Recently I wanted a similar setup for my own '11 Macbook, and after much research and trial/error, I was able to get it to work.

The problem is not your machine, Mac OSX, the boot camp installer, or even your Windows installation disks. The fact is that Windows itself cannot be installed from an external CD drive, nor can it be installed on an external HDD. The reason why you don't hear much about this problem is because most people with Macbooks are installing their boot camp partition on the same HDD as their Mac OS, with the built-in superdrive. For us who want to run 2 HDDs in the Macbook, with one of the drives being the boot camp drive, we make the honest mistake of taking out the superdrive, thinking that we can just install Windows from an external optical drive. Jus know that this will never work.

So if you want to run 2 HDDs from your Macbook, with one being for boot camp, the steps below worked for me:

- Take out the MCE optibay and put back the superdrive into its original location.

- Install the drive that you want to install boot camp into the original HDD drive bay.

- Stick the original OSX install disk into the superdrive and first install Mac OSX onto it. Realize that you are only doing this to run the boot camp install and will be wiping it out later.

- After you have installed OSX, go through the initial setup and be at the desktop. Run the boot camp assistant and go through with the install and have it create a partition for boot camp. At this point, it doesn't really matter how big/small the patition is for Windows. You can adjust and resize the partition during the Windows install process for choosing the location and partition.

- Go through finishing the boot camp assistant in OSX, stick your Windows install CD into the drive and boot into it. This time it should work.

- Once you have completed the Windows installation and you are at the Windows desktop, stick the Mac OSX cd back into the drive and run the setup.exe. This will install all the drivers that will make it recognize all the Mac hardware, etc.

- Finally, take out the CD drive, swap back in the optibay, put your boot camp HDD in there, and put back the HDD with your Mac OS.

- That's it. Now when you boot up, the EFI boot will recognize the Mac OS and Boot Camp and you are on your way.

Hope this helps.

Same way I had to do it except what you did in bold is a HUGE waste of time...bootcamp assistant does nothing other than format a partition of your specified size to a format that the Windows Installer can read, FAT32(You can format it as HFS or HFS+ but it becomes a pain when your trying to find which partition to delete/format in Window so do it as FAT32 for simplicity). Just format a partition on the target drive while in OS X normally to FAT32, shut computer off, move HDD to stock spot, put superdrive in. Start computer put disc in and hold option, select cd, format partition using Windows installer and install windows, do all windows updates and then install bootcamp updates and then shut computer off and move drives to position you want.
 
Same way I had to do it except what you did in bold is a HUGE waste of time...bootcamp assistant does nothing other than format a partition of your specified size to a format that the Windows Installer can read, FAT32(You can format it as HFS or HFS+ but it becomes a pain when your trying to find which partition to delete/format in Window so do it as FAT32 for simplicity). Just format a partition on the target drive while in OS X normally to FAT32, shut computer off, move HDD to stock spot, put superdrive in. Start computer put disc in and hold option, select cd, format partition using Windows installer and install windows, do all windows updates and then install bootcamp updates and then shut computer off and move drives to position you want.

I wouldn't call it that big of a waste of time:) But looking back on what I did then and what I know now, I agree with you that it is not necessary. More or less these are the step by steps for getting this 2 HDD setup with Bootcamp on one.
 
I wouldn't call it that big of a waste of time:) But looking back on what I did then and what I know now, I agree with you that it is not necessary. More or less these are the step by steps for getting this 2 HDD setup with Bootcamp on one.

Snow Leopard takes about 30 minutes to install, quite the waste of time if you ask me when the whole process for moving drives and installing windows can be done in about 45 minutes total...
 
Snow Leopard takes about 30 minutes to install, quite the waste of time if you ask me when the whole process for moving drives and installing windows can be done in about 45 minutes total...

Won't most people have snow leopard (or some version of os x) installed anyway??
 
Won't most people have snow leopard (or some version of os x) installed anyway??

Did you read my posts? It is a waste of time to install Snow Leopard on to the drive just to create a partition when that can be done with both drives installed, you can even have the new drive in without OS X, run the Snow Leopard DVD and just go into Disk Utility without installing Snow Leopard and format as you desire...no reason at all to go through all the time that it takes to completely install the OS.
 
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