Generally, the installation needs to be on your main hard drive.

While the data files could go on another drive, I think the data files are fairly small.

Fifteen years?  Do you EVER go back more than six years?  If not, uninstall the older versions.

Hard drives are fairly low-cost; you would be better off finding someone that could transfer EVERYTHING to a new, larger internal hard drive.  Or if your computer is fairly old, replace your computer and make sure your new computer has a larger hard drive.