FreeFileSync Forum

Any fresh installation selecting portable checkbox won't have "install.dat" file.
You may test it yourself. (Make sure you don't have any existing installation on the computer).

Meaning we can only make a portable installation 1) on a USB drive and and 2) only if it's not installed on the hard ...

Added in version 8.0, among others:
"Automatically check consistency of FreeFileSync installation".