PS6000E Disk activity light

Just got an old PS6000E off the shelf and up and running - during the build process I had 6 disks error - these were replaced, the RAID finished rebuilding but I am noticing that the disk activity light for each disk will blink very quickly - this happens on each disk in turn, goes on for several hours, then on to the next disk. (only happens on one disk at a time)

I honestly never noticed this before because our SANS are typically remote to where I am.  I assume this is normal - either an index activity, or a disk health check.

Just wondering if anyone knows what this actually is so I can decide if I want to interrupt it in order to rack the SAN.