To do that you will have to set the path to the name of the flash drive. For instance mine is G:\ when it's plugged in the port.
So it would be
#################
## User Config ##
#################
ACTIVE_DIR=r"YourDriveLetter:\Install PathToPref_1 Folder\pref_1\brushes" #<<<-----SET ME<<<----- (Directory to hold active sets, select Writable in Preferences)
INSTALL_DIR=r"YourDriveLetter:\Install PathToPref_1 Folder\pref_1\gimp\brushes"#<<<-----SET ME<<<----- (Directory where you place your sets - in subdirectories)
EXTENTIONS = [".gbr", ".vbr", ".gih", ".abr", ".GBR", ".VBR", ".GIH", ".ABR"] # Valid brush extentions
################
## End Config ##
################
The manager will then grab the brushes from the USB drive itself out of the pref_1\gimp\brushes directory and place them in pref_1\brushes directory and refresh.
That should work for porting.