Failed to add disk ... Reason: Hot-add of digest enabled disk not supported

Troubleshooting

One that needs specific attention is the error, "Hot-add of digest enabled disk not supported".  This means specifically that View Storage Accelerator (VSA) has been enabled (also called Host Caching).  VSA causes a set of additional files to be associated with every VMDK it affects: diskname-digest.vmdk and diskname-digest-flat.vmdk.  As the error says, you cannot "hot-add" a digest-enabled disk, which means you cannot add a digest-enabled disk to a running VM.

Unfortunately, Unidesk rebuilds require being able to hot-add disks to the CP all the time.  Requiring a CP reboot every time we needed to add a disk would be a disaster.  So enabling VSA, and thus enabling disk digests, cannot function with vRangerPro 7.

If you are getting this specific error in a Reconfigure task in vSphere, then you need to disable View Storage Accelerator on the VMs pools.  It should be sufficient to disable VSA on the VMs pools, so try that first.  Otherwise, you would need to disable it as the vCenter level in View.
First, select your VMs Pools in the View Administrator page and Edit them.  In the Advanced Storage tab, look for Host Caching and/or View Storage Accelerator, and disable it.  That should stop VSA from making your desktops any worse, though you might need to stop and start them again to allow View to properly reconfigure them.
Specifically how to do this depends on the version of View (or Horizon View) you're using.  For instance, this URL describes turning it off for View 5.1.
http://blogs.vmware.com/euc/2012/05/view-storage-accelerator-in-practice.html

In View 5.2 and beyond, the feature is no longer called Host Caching, but is explicitly called View Storage Accelerator.
After you have disabled VSA on your pools, go to vSphere and find the CachePoint that the bad desktop is on.  (You may need to do this on multiple CPs.)  Use the vSphere "Browse Datastore" function and go into the CachePoint folder.  
Look through all of the layers (OS, App and User) and look for files named something-digest.vmdk and something-digest-flat.vmdk.  Simply delete those.  The presence of a digest file is how VMware knows that a VMDK is digest-enabled, so removing an leftover files is necessary.

Or shutdown all VMs after you have disabled VSA.
If it has been properly disabled, vSphere will delete all digest files.
If disabling VSA at the pool level and scrubbing out the leftover files isn't sufficient, you will need to turn VSA off at the vCenter level.  In View Administrator, go to View Configuration, Servers, and edit your vCenter.  Turn off Host Caching/VSA there, and then go back and search for the digest files again.
If you still get an error after this, contact Tech Support to see if we can get VSA untangled from the desktop VM (since it would be gone from the layers by this point).