ftblaunch.cfg uses an absolute path for installPath. I would like to have it portable and used '.' instead.
However, when launching an MC instance it then crashes with the following log.
Maybe expand the installPath before launching.
Workaround, but not a solution:
Removing the installPath line from the ftblaunch.cfg before launching the launcher, will allow you to make the installation portable.
If you're on windows, this can be achieved by creating a batch file like this:
type ftblaunch.cfg | findstr /v installPath > portable.cfg
ren portable.cfg ftblaunch.cfg
start javaw -jar FTB_Launcher.jar