Hi
I have VNX5400 . 3 days ago,one of my vault disk failed(third one) ,  after some hours , second vault disk failed(forth one) . i replaced 2 disk , and now rebuild is completed , all errors disappeared but one warning that say :

The storage system has disabled write cache.
The write cache on the storage system has not been configured yet or has been disabled because of a hardware component problem or software problem.
Verify that system memory has been allocated to write cache and that write cache has been enabled. Also, verify that the SPS ac power and serial communications cables are connected properly between the storage system and SPS. Replace any faulted hardware components and wait several minutes for write cache to automatically enable after these faults are cleared. Contact your service provider if the problem persists.
0x720f

But when i try to re-enable write cache manually i see error that say :

Enable write cache - Write Cache is not enabled due to any of these reasons:
1. There is no Standby Power Supply (SPS) present.
2. One or more LUNs are offline.

of course no power or no sps or no power cable or anything like that failed !
i'm not see any problem to my LUNs , and all of them are Online and enabled .

this problem happened after vault drives failed and now it seem they rebuilt , what is the problem ???
Thank you in advance