Regarding the problem of unmounting a datastore, there is a known issue in VC 5.1 that was fixed in VC 5.1U1. In 5.1, in certain circumstances, during the datastore unmount operation, VC is unable to log into the HA agent on the host on which the unmount is being done, and hence the HA agent's use of the datastore cannot be quiesced.
You can determine if you are encountering this issue by searching the VC log files. Find the task corresponding to the unmount request, and see if the follow error message is logged during the task's execution:
2012-09-28T11:24:08.707Z [7F7728EC5700 error 'DAS'] [VpxdDas::SetDatastoreDisabledForHACallback] Failed to disable datastore /vmfs/volumes/505dc9ea-2f199983-764a-001b7858bddc on host [vim.HostSystem:host-30,10.112.28.11]: N3Csi5Fault16NotAuthenticated9ExceptionE(csi.fault.NotAuthenticated)
While we are on the subject, I'll also mention that there is another know issue in VC 5.0 that was fixed in VC5.0U1 (the fix is in VC 5.1 too). This issue related to unmounting a force mounted VMFS datastore. You can determine whether you are hitting this error by again checking the VC log files. If you see an error message such as the following with VC 5.0, then you may be hitting this problem. A work around, like above, is to disable HA while you unmount the datastore.
2011-11-29T07:20:17.108-08:00 [04528 info 'Default' opID=19B77743-00000A40] [VpxLRO] -- ERROR task-396 -- host-384 -- vim.host.StorageSystem.unmountForceMountedVmfsVolume: vim.fault.PlatformConfigFault: