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How to Remove Document Services, Dropbox, Box, Google Drive
If you are wanting to login to a linked Document Service as another user account here is how to do this in each of the Document Services Workfront supports.

Google Drive:
Login to your Google drive > Click the Settings Gear in the upper right > Select “Manage Apps” > find Workfront Inc. in the list > choose the options drop down > Disconnect from Drive.
 
If a you would like to change the Google Drive account associated with your instance of Workfront, you will need to perform the following:
 

Box:
Login to your Box account. Click your Name drop down > Account Settings > Security tab > find Workfront and click the X under Forget App.

Dropbox:
Login to Dropbox. Under your User Name in the Upper right > Settings > Security tab > scroll down towards the bottom > Apps Linked > click the X next to Workfront.


Additional Information:
Workfront requests permission from the User to access their document service (Box, Dropbox or Google Drive)
The user provides their document service credentials in order to login.
The document service "Links" itself to Workfront.

Since the document service is the one allowing permission to link to Workfront, it is not possible for Workfront to remove the permission granted by the document service. You need to remove the permission from within the document service application (see above on steps)
 
Workfront: 
Go to Documents tab > Click on Add Documents > Look for Google Drive in the left side and it it isn't there then look in the ADD NEW SERVICE area and select From Google Drive > click ReAuthenticate > This will grant access to all of your documents that you have access to with Workfront. 
 
 
 *Within workfront you will not be able to remove the name of the service that comes up in the left hand side of the screen when you click on the "Add Document" button. What you can do if there is one you don't want to use is you can click on the little dropdown arrow to the right of the service (once you have clicked on it) and then you can select "Rename" to give it a name that tells you not to use it.