The vast majority of portable AutoCAD files on torrent sites or forums are bundled with trojans, keyloggers, or cryptominers. Google Drive’s virus scanning is basic. Downloading a pre-made "AutoCAD 2015 Portable" from an unknown source and syncing it to your Drive is a security nightmare.
Disclaimer: The following instructions are for educational purposes for users who own a legitimate license and wish to explore portability. Do not use cracked software.--- Autocad 2015 Portable Google Drive