Access all user drives as Admin - microsoft-graph-api

Question

I am building a native application that uses Microsoft Graph.
I want to read all the files in OneDrive of all the users in the organization, via admin account.
I am using the (Azure AD 2.0) OAUTH authorization workflow to get the authorization code. After which I get the access token and refresh token.
However when I try to access the drive of any user :
graphClient.Drives["amit#csys.onmicrosoft.com"].Root.Request().GetAsync();
// where graphClient is instance of GraphServiceClient
I get:
generalException
Message: Unexpected exception returned from the service.
With call stack :
at Microsoft.Graph.HttpProvider.<SendAsync>d__19.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
at Microsoft.Graph.BaseRequest.<SendRequestAsync>d__34.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
at Microsoft.Graph.BaseRequest.<SendAsync>d__32`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
at Microsoft.Graph.DriveItemRequest.<GetAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at OneDrive_Writer.OneDriveWriter.<loadRootDriveFolder>d__9d.MoveNext()
Is there a way an admin can access all other users drives files?
NOTE: I am able to access the Admin's OneDrive account when I login as Admin.

Answer 1

The Drives indexer in your example expects a driveId, not a upn.
Assuming that your permissions are properly set, and you have a valid access token,
you'll do something like this.
// Access the users in the org.
var users = await graphClient.Users.Request().GetAsync();
// Get the drives for a user.
var drives = await graphClient.Users[users[0].Id].Drives.Request().GetAsync();
// Get the specific drive metadata for a user
var drive = await graphClient.Drives[drives[0].Id].Request().GetASync();