Possible condition below. There was a bug in 5.1 I think as well, I'd have to dig up the KB.
Unmounting or Removing Datastore Fails
When you try to unmount or remove a datastore, the operation fails.
Problem
The operation to unmount or remove a datastore fails if the datastore has any opened files. For these user operations, the vSphere HA agent closes all of the files that it has opened, for example, heartbeat files. If the agent is not reachable by vCenter Server or the agent cannot flush out pending I/Os to close the files, a The HA agent on host '{hostName}' failed to quiesce file activity on datastore '{dsName} fault is triggered.
Cause
If the datastore to be unmounted or removed is used for heartbeating, vCenter Server excludes it from heartbeating and chooses a new one. However, the agent does not receive the updated heartbeat datastores if it is not reachable, that is, if the host is isolated or in a network partition. In such cases, heartbeat files are not closed and the user operation fails. The operation can also fail if the datastore is not accessible because of storage failures such as all paths down.
Note
When you remove a VMFS datastore, the datastore is removed from all the hosts in inventory. So if there are any hosts in a vSphere HA cluster that are unreachable or that cannot access the datastore, the operation fails.
Solution
Ensure that the datastore is accessible and the affected hosts are reachable.
Reuben Stump | http://www.virtuin.com | @ReubenStump
