I routinely boot Win 7 from at least two drives, and ran into a problem with Google Drive refusing to sync, saying the Google Drive folder was missing. My folder is on a "data" drive, separate from the OS drives. A search revealed others with this issue. None of their solutions worked for me, but did lead me to a solution:
In "OS1", unplug the network cable and delete all files from the Google Drive folder. (will get a message saying files have been moved to Trash at drive.google.com, but this obviously cannot be true at this point)
Boot "OS2", attempt to use the Google Drive folder - will not work, but will show disconnect option. Disconnect, then sign in, go to advanced setup, and specify the same (empty) Google Drive folder used by "OS1" (key: this will NOT be allowed unless the folder is empty).
Choose "start sync". Files will download & folder will get Google Drive icon.
Google Drive will now function in both "OS1" and "OS2" using the same folder.
