The magic is done in scripts that you use to start GRAMPS. The scripts trick GRAMPS into thinking that your user home directory is on the removable drive instead of in the usual place, so that GRAMPS will read and write its configuration and data there. The process is mostly transparent, but you will probably need to edit the media base path each time you run GRAMPS on a different computer. You may also need to edit some settings in the scripts for different computers. If there are certain computers which you use regularly that need different settings in the scripts, you can easily make separate script copies for each one, to avoid the need to reedit them.

Save the scripts below in the directory you want to use as your GRAMPS home directory on the removable drive. When run, the scripts will direct GRAMPS to the directory they are installed in. Edit the script SETTINGS section of the scripts you will use, so that the values in the SETTINGS section there are correct for your computer. You will probably need to change the media base path setting in GRAMPS each time you run it on a different computer. You may also need to edit some settings in the scripts for different computers. If there are certain computers which you use regularly that need different settings in the scripts, you can easily make separate script copies for each one, to avoid the need to reedit them.