How to stop Windows Installer from incorrectly installing files to the larger of two or more drives

I've created a reasonably simple MSI; essentially all it does is install a startup script, activate it, and copy two necessary files to C:\Temp\Updates.

Everything works fine... except when I run the MSI package on a machine on which the E:\ drive is larger than the C:\ drive.  In that case, the files destined for C:\Temp\Updates end up in E:\Temp\Updates.

I've tried installing with several variations of the following command line:

start /wait msiexec /i "T:\software\MSI\StartupScript_1.0.msi" /TARGETDIR=%systemdrive%\ /l* C:\msilog.txt /passive

I posted a question on this previously and got an answer: my current solution is to use a custom action that redefines [INSTALLDIR2] as "C:\Temp".  However, I sometimes work with machines that do not use C:\ as their system drive, so coding this without a variable makes me uneasy.

In my previous question, one expert (Merowinger) suggested that I attempt to find the property in the MSI that triggers the check for free space.  This struck me as a better solution, but I could not identify the property (there is a "Free Space" property, but it seems to be checking for registry space or something).  Can someone identify that property for me so I can set it to 'disabled'?

Cheers,

TDR