How to do it

Navigate to the root RimWorld folder, which should be '*\Steam\steamapps\common\RimWorld\', however you can find this by going to your Steam library, right clicking RimWorld, clicking 'Properties', going to the 'Local Files' tab and clicking 'Browse Local Files'.Move the 'Rimworld' folder to your USB drive (from */steamapps/common/)Open notepadPaste the below commandstart RimWorldWin64.exe -savedatafolder=SaveDataClick File > Save AsChoose a file name and add .bat at the end, like 'Launch.bat'Select 'All files' in the 'Save as type' drop downSave the fileNow put the .bat file into the root of your RimWorld folder is, where the 'RimWorldWin64.exe' file isLaunch RimWorld from your portable file using the .bat fileNow your save data should go into that file under a folder called 'SaveData', you can edit this by altering the 'SaveData' in the .bat fileUsing this method will also support mods as long as you put them in the 'Mods' folder in your portable RimWorld locationTo move Steam workshop mods, navigate to the file path of RimWorld's workshop file as shown below:*\steamapps\workshop\content\294100\Then take all of the files (should be a some random numbers) and copy them to the 'Mods' folder in your portable RimWorld folder.NOTE - If Steam is not installed on the host PC, it should warn you that RimWorld is having trouble with the Steam API, just click ignore and it should run normally (without Steam)