Why can't I start my VM when there is plenty of free memory? Issue #1: A number of customers has reported than VM's fail to start when there is plently of memory. The error reported from the Hyper-V manager is displayed below. Here I tried to start a 4GB VM when there was 7+GB free.
The most common cause has to do with a bug in Windows Server 2008 NUMA memory allocations. This means you'll only hit this bug on NUMA machines (all multi-proc AMD machines are NUMA + many high end Intel based servers). You can verify you have this bug by looking at the Task Manager reported "Cached" and "Free" memory.