Hey guys, just wanted to update the thread with the "resolution" that worked for me...  It looks like it is a memory overhead shortage on my ns-120.  Pool luns require a certain amount of memory per lun, so even though I have plenty of capacity on the disk side, I must have run out of memory on the SP allocated to pool lun overhead.  I found that I had a bunch of RLP luns inside my pool.  I removed one and delted it and found that I had the capacity to create one more LUN.  So apparently it looks like their is a flat cost per LUN regardless of size.  I just deleted 5 RLP luns and now I have capacity to create my new LUNs.  I am going to mark sylvain-20x4-'s post as helpful since my understanding of the EPC enabler is that it is somewhat of a patch that allows you to steal memory allocated to ther features and allocate it to the pool overhead.  This would probably have also resolved the issue.  Thanks to everyone for their time.  Hopefully this will help someone else before they dump their NS-120 / Clariions.
I have an NS-120 (cx4-120) and I am trying to create a 50 GB LUN.  There is 17 TB unused capacity and we are no where near a max LUN count.  I am trying to create a LUN in our one and only POOL0.   Any ideas why I would get this message?  I have read through some threads that mention using the EPC Enabler.  Not sure what that is.   I am creating this LUN to be used a secondary mirror of a lun on our new VNX 5400.  Yesterday I was getting the same error when trying to create the LUN using the mirror wizard so I had to unisphere directly to the ns-120 and create the LUN first, then designate it as the secondary mirror.  Today I get the error creating the new LUN directly from unisphere connection to either SPA or B.  Driving me nuts!!  Thanks,  C
LUN Could not be created: ...it would have left the array operating over its 
Safe Operating Capacity