It is a starting point but the problem is really not with whether there is enough space to download installation files, for they can be downloaded remotely to some other disk. The problem is when installing from the downloaded files, the system itself may give an error saying no disk space left. The problem is when installing the file I presume some files are written in linux directory usually I presume or guess in /bin/ or /sbin so that the installed programs become usable. When an external disk is added, it is writable and readable but its space does not become incorporated or available to /bin /sbin or whatever directories in linux filesystem get used... Is it possible to make some changes to filesystem hierarchy so that the additional disk becomes available to the system?
It is possible with some programs... One could extract files to any directory one wishes, and run the program from the extracted directory. I am not sure if it is possible with any program. (are .deb files basically compressed files?)
I guess one problem one could run into is when the program needs to read from and write to the rest of the system it needs to know where it is located and how to access other elements of the system...
But apt-get install or dpkg -i will install files to /bin or /sbin ... Do they have an option to install elsewhere and be able to resolve other problems such as knowing its path and being able to communicate with other paths appropriately?