Because I want to run TeXnicCenter and MikTeX on an USB-Drive, I have to variable the drive letter within the output profiles. Is there any possibility? The TeXnicCenter-placeholders only have a placeholder for the drive letter of the project main file or the current file. And even this one ("%rm\") did't work.
As far as I know there is no such an variable for the drive letter. To get it a changing of the code of TeXnicCenter is needed.
All windows programs at last have to use the registry for parameters for the usage of the program. The worst case would be that you have to change one or more of the parameters inside the registry.
At last I suggest you to visit the homepage of TeXnicCenter and go to the support page. Make a feature request and ask there for adding such an parameter you need.