Boot Camp sees my internal SSD as an external drive - can't install Windows 7

I've just replaced my old 1TB drive on my Mac Pro (mid 2010) with a 1TB Samsung 840 EVO. To get the best speed I've installed it using the Sonnet Tempo PCIe interface. I performed a fresh install of Mavericks 10.9.1 and the system absolutely flies.

However, I wanted to also install Windows 7 but Boot Camp Assistant will not allow me to do this. It warns....

You cannot partition an external disk, a RAID disk or an internal disk on which FileVault encryption is in progress.

When I look through the system information, my SSD is reported as an external drive - which is why I guess BCA refuses to partition it (and ghosts out the option to install Windows 7 or later version.

Here's the system information under storage...

MAC SSD:

Available: 941.47 GB (941,471,612,928 bytes)

Capacity: 999.35 GB (999,345,127,424 bytes)

Mount Point: /

File System: Journaled HFS+

Writable: Yes

Ignore Ownership: No

BSD Name: disk0s2

Volume UUID: DED0438B-7A12-3AB0-8E6D-E6911C595CA2

Physical Drive:

Media Name: Samsung SSD 840 EVO 1TB Media

Medium Type: SSD

Protocol: SATA

Internal: No

Partition Map Type: GPT (GUID Partition Table)

S.M.A.R.T. Status: Verified

So my questions are... Why does OSX identify it as an external drive? Is it because of the PCIe interface? And, more importantly, how can I get around this to allow me to install Windows 7? Could I temporarily remove the PCIe card and install the SSD in a standard drive bay and then run BCA? Any ideas???

Thanks.