Boot Camp Assi doesn't like my external system drive

Windows cannot be booted from nor installed on external drives. That is the reason for the error.

Yes, but Boot Camp Assistant will only partition the internal drive if it has OS X installed on it or another internatl drive. In your case you have only one internal drive, so you must have OS X installed on it to use Boot Camp Assistant.

What I would suggest is that you open Boot Camp Assistant then click on the large Print Documentation button. Read the documentation carefully.

You will have to ask Microsoft. This is a Windows issue, not one of Apple or Boot Camp Assistant.

If you search Google you will find there are mechanisms for getting Windows to install from a USB device, but you will need access to a Windows computer to implement them.

Windows very rarely installs unless the port is on the motherboard itself or the controller is one which has a Windows boot init.

You can clone windows here and there and might get the same result in the end.

Or swap hard drive places.

But you can boot from external and use the internal drive - it either has to see the drive as 100% MBR or you have OS X present on the internal drive also.

It really should work. The docs are not written with dual drive Mac Pro and others in mind: iMac w/ SSD and 2TB hard drive and now dual drive MacBook Pro with two SSDs etc. where both drives cannot be GPT/GUID as Windows won't install in those cases either.

I'm afraid you misunderstood. Ask MS why Windows won't install on or boot from external drives. Has nothing to do with Boot Camp.

No need to be rude.

you can. you can even just boot from the DVD by itself, format and use 100% internal drive.

It does work.

BCA is or use to be called a "one trick pony" that would only do it one way / it's / and even if you had 4 internal drives there were unwritten 'rules' of what and how.

But totally doable. BCA is ONLY required for:

downloading support drivers

partitioning where OS X is and will be for a dual-boot two partition setup.

Otherwise it is optional or even not needed.

I would have to point to the You don't need to do anything of my comments above.

You don't partition the drive with Disk Utility. No purpose.

You can if you want and in that case set the drive partition table to Windows Master Boot Record and devote the drive to a single FAT32/MSDOS volume if you must.

You will be formatting it later in Windows Installer and you can have one or more NTFS volumes.

This is a Windows only drive.

If you want to have an HFS+ volume, you can convert or format or use Paragon's utility that can convert while maintaining data HFS <=> NTFS for those that had data that they need to maintain. Or install HFS and NTFS drivers for both operating systems.

And we have been saying 50/60 and up for Windows 7. Not 20GB which is what the actual core OS itself uses, does not take into account page, hibernation, temp, cache, apps, data, thumbnails, and the 10GB+ for all the initial updates to be applied. Or the GBs that are needed to create and setup each user account. (You won't be able to logiin on next restart with 20-25GB). And no room for those handy "restore points" that need a couple GBs.

Now, if you are ONLY using a small 50GB on a drive, before answering, how do you plan to actually use the drive? HAS to be answered first.

I don't endorse this but it was educational and well written write-up:

Successful setup of OS X Lion + Data Partition + Bootcamp Win7 Ult

https://discussions.apple.com/thread/3293948

I have never seen W7 even 32-bit take 12GB and MS lists 16GB minimum - and most of those that try have installed and then done updates, and only then cloned the OS removing what they can to trim down a normal install. A VM can be much smaller and maybe half. Not a native install and setup - no page file can also be unstable.

I tend to look for rules and find that threads like ours often only get the details at the end, not up front of what is behind the need or problem or question.

But you know how. And how you use the rest of the drive.

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1. First Boot Camp Limitation – Free Space Rearrangement

Apple released its famous utility for dual-system configurations on a Mac – Boot Camp, which shrinks HFS+ volumes in Mac OS X and prepares a partition for Windows right alongside. One problem with Boot Camp is that you cannot resize these partitions later. If free space on one of the partitions is reduced to critical levels and other volumes still have a large amount of unused space, you get into trouble.

The solution for this situation is to relocate space between volumes with Paragon’s CampTune utility. CampTune rearranges free space between HFS+ and Windows partitions and shrinks existing volumes to free unallocated space on a hard disk, and create new partitions without the need to use Boot Camp at all.

2. Second Boot Camp Limitation – FAT32 Only

Apple Boot Camp does not support the creation of NTFS volumes because Mac OS X does not have the appropriate NTFS driver. Since Windows Vista and Windows 7 can be installed only to NTFS volumes, you will have to reformat this partition during the installation.

An easy alternative is to use Paragon CampTune from the beginning: resize the primary HFS+ volume, create second partition, format it as an NTFS volume and install Windows directly.

3. Accessing NTFS Volumes in Mac OS X

Apple’s Mac OS X operating system has limited support for NTFS volumes; the NTFS driver in Mac OS X can read data only. You cannot change files, copy them to NTFS volumes, create new files, or NTFS volumes – all significant limitations.

However, you can easily bypass these problems with Paragon’s NTFS for Mac OS X driver. It has none of these limitations and provides the same level of NTFS volume compatibility for Mac OS X as it does for regular HFS+ volumes. You can easily manage data on any NTFS volume; create files and folders, and copy data between HFS+ and NTFS partitions. The driver adds formatting support to Mac OS X for NTFS file systems and is accompanied by set of utilities for NTFS volume management, checkup and repair - effectively removing boundaries between two incompatible yet universally used file systems.