Correct, and by installing Windows, that portable must be bootable by whatever system it gets connected to, and it eats up your file storage space in the process. In short I don't think one achieves the desired end state with this configuration.
Well, for my specific case, I'm basically wanting a laptop's portability without wasting thousands of dollars on functionality that (it seems to me) can be feasibly reproduced in other ways.
If I boot one drive, I can still read the contents of another, though correct? Such as the case where I boot my portable which is connected to the computer via firewire/usb (though I would not choose a USB). I can view the contents of the Drive "C:" which would be the hard drive of the main tower, right? Whereas the portable would be something like X: or Q: or something...
hmph.
So if I install, for instance, Autocad 2009 on the main tower's drive... then 3dsMax on my portable... I couldn't run them both at the same time, and could not switch between them without rebooting from the other drive... that is rather unfortunate... unless I dual boot? I'm not familiar with that scenario though, so I dunno..