Unfortunately all of our programs need to be installed on the same drive as the operating system. You will not be able to install on any other drive other than the C: Drive. You will not see an option to choose an alternate drive to install the ON1 Photo RAW during the installer.
However, please note, the installer for our programs only takes about 1.5 Gb of HD space in order to install. You will be able to free up space on your C: drive and access all of external/network drives from within the Browse module of the Photo RAW if you are running low on disk space.
Unfortunately we do not have any immediate plans to add the ability to install on any other drive.