Utility or trick to get screenshots from USB-Adapter-driven monitor?

Question

My main computer is a G5Quad, but this post relates to my MacBook.
Recently, I acquired a USB-(display card)Adapter ( http://eshop.macsales.com/item/NewerTech/VIDU2DVIA/                  ) to run the second one of my dual external monitors with my MacBook (not "Pro"). 
The first one of the external monitors was already connected to the MacBook through the regular external-monitor port, so this allowed for the use of both external monitors on the MacBook.
To my utter astonishment and delight, the arrangement works absolutely superbly. I ended up with an arrangement in which my desktop spans all three monitors, with my main monitor on the right (22" CRT) being the one connected directly to the MacBook's monitor port,  the center one (also a 22" CRT) conected through the above described USB adapter, and on the left the MacBook's own built-in monitor.
Working with Photoshop13 ("CS6") with all that screen real estate is like a dream.
The only thing that doesn't work is taking a screen shot of the center display, i.e. the one connected through the USB adapter.  Using Command_Shift_3 generates only screen-shot files from monitors one (right) and three (left), the center one is simply
ignored.
Using Command_Shift_4 only in the center screen, I can hear the shutter-clack sound, but no image is generated again.  I've tried swapping the screens and connections around, and whichever is connected through the USB adapter is always ignored.
If anyone can suggest any trick or knows any utility that will help me get this done, I'd be much obliged.
Running Snow Leopard 10.6.8, and here are the MacBook's details:
Name: MacBook (late 2006)
ModelCode: mb_late_06
Model Number: MA701
Machine Model: MacBook2,1
Model introduced: 2006
Factory: W8 (Shanghai China)
Production year: 2007
Production week: 18 (May)
Production number: xxxx (within this week)
CPU speed: 2.0GHz
Family name: A1181
Screen size: 13 inch
Screen resolution: 1280x800 pixels
As per Apple System Profiler:
Hardware Overview:
  Model Name:    MacBook
  Model Identifier:    MacBook2,1
  Processor Name:    Intel Core 2 Duo
  Processor Speed:    2 GHz
  Number Of Processors:    1
  Total Number Of Cores:    2
  L2 Cache:    4 MB
  Memory:    4 GB
  Bus Speed:    667 MHz
  Boot ROM Version:    MB21.00A5.B07
  SMC Version (system):    1.13f3
  Serial Number (system):    WXXXXXXXXXX
  Hardware UUID:    00000000-0000-1000-8000-001B631BC56F
  Sudden Motion Sensor:
  State:    Enabled
Both external CRTs are running at 1280x1024 pixels
Thanks in advance.

Answer 1

Found the utility I was looking for:  SnapzProX2 does the job perfectly, by either taking a screen shot of just the secondary (center) monitor or one spanning the entire width of all three screens combined.
Alas, the utility costs $70.    I'll continue looking hoping to find something cheaper.
However, this proves that the USB-adapter is indeed working flawlessly and that it provides all the necessary information to capture a screen shot of the monitor it is driving.
So this is really an Apple problem, which I have no hope of seeing fixed, as Snow Leopard is no longer supported.
If somebody finds something else, please let us know.