I went through this exact same thing on one of our CX700s in the last couple of months. Here is what happened:
A disk had faulted and was being rebuilt to a hot-spare. When the disks in the raid group were being read from to rebuild the failing disk to the hot-spare, it encountered a sector read error, but no-longer had all the parity and data to rebuild with (hence, it was actually a double-fault at the data-stripe level across the raid-group). When Flare encounters this condition, it has no choice but to mark the sector as invalid.
This situation is typically a hardware problem, but there are software tools in place to minimize the risk of this happening. A “sniffer” program is incorporated into the Flare code that continually scans each lun to insure it can read all sectors and that data and parity calculations match. If the sniffer encounters an error, it attempts to remap the sector. Release 19 of the code has vast improvements in the way sniffer operates, allowing the sniffer to run at a much faster rate without impacting the hosts.
In our situation, we were extremely lucky that the affected LUNs were not in use because they did need to be unbound and rebound, and then EMC had to actually physically reseat the hotspare and the original failed disk before we could bind and use the LUNs again.
What version of FLARE are you on? I am guessing it is pre-19, and if so, you need to upgrade to 19 as soon as possible especially if you have ATA drives in your Clariion.