Cannot see new drive in SSMS maintenance wizard
+1 for sensibly wanting to get the backups on a separate drive. However, if the new cluster volume is on the same SAN, you're not really accomplishing much. It would likely be much better to run backups to a file share on a totally different system. 

As a workaround, you can easily add an "Execute T-SQL Statement" task to your Maintenance Plan to manually specify the backup statement: