Fixing a storage space that does not attach on reboot

Recently one of my Hyper-V servers developed a bad problem.  Whenever it was rebooted it would not attach the storage space that contained all of my virtual machines.  The result was that every time the system started up again - I had to connect to the server and manually mount the storage space before any virtual machines would start.  Clearly - this was not acceptable.

Having a look at the storage space with PowerShell quickly revealed the problem.  The virtual disk was marked as "manual attach".  I do not know how this came to be, but I suspect it is the result of one time when I had to replace a failed disk (I am currently chatting with the Storage Spaces team to try and figure out how this happened). 

However, having located the problem - it is easy to fix:

I used Set-VirtualDisk to change "IsManualAttach" to false.  A quick reboot confirmed that the disk was now mounting automatically.