Well, because with the FREE version you are allowed to use only one cloud/location it's necessary to disable OneDrive first.
Due to Boxcryptor's autodetect feature and because OneDrive is the "standard" cloud on Windows (and it can't be uninstalled), OneDrive will be always first activated.
Now, to solve your problem.
You have already copied teh .bc files from OneDrive.
Install the Google Sync software, sign in, activate the sync feature.
Copy your files from the USB drive to Google Drive.
Go to Boxcryptor settings, "Locations" tab and deactivate OneDrive, activate Google Drive.
Keep in mind, that you have to deactivate OneDrive again in case you sign out from Boxcryptor in the client (settings will be reverted to defaults, so OneDrive is again the default location).