Error creating database on new storage in SQL Server 2008 R2 Failover Cluster
Operating system error 5 is a simple Access is denied you can always check those errors in the command prompt net helpmsg 5. 

Now Volume mount points are sometimes a bother. In Windows server 2000 the calcs.exe had an error and could not set the permissions on the underlying drive and the only way to do it was to mount the volume using a drive letter, set permissions on the root folder and then create a mountpoint. 

Later versions of windows should be able set the permissions on the mountpoint by using the /M switch so that


  cacls j:\data /g DOMAIN\ACCOUNT:f /M /E


Will grant the domain\account full permissions on the root of the drive mounted at j:\data, I still haven't leard to do this with icacls.exe and a comment how to do so will be well appriciated. 

As you have found out creating a subfolder on the mount point makes it possible for you to create an accesslist for that folder but there still seem to be issues with setting permissions on the root folder of the drive using the GUI.

Usually I dont bother with the calcs command and if I need to set permissions on the root I'll add a driveletter, set the permissions using the GUI and then remove the drive letter.