Hard Drive Array issues

So, I have an Icy Box IB-RD3640SU3 4 Slot RAID array, setup with 4 Hard Drives in RAID 5.

Usually, my Hard Drive array has an Orange indicator on the RAID 5 setting, with all 4 HDD indicators Blue / Flashing Purple Blue when they're accessed.

Today, HDD 3 seems to have seemingly randomly switched to "Flashing Red-Blue (Maybe Purple)" Mode, whilst all other drives are on "Probably flashing Purple-Blue, but so fast it looks like Purple" mode. There also seems to be a slightly less bright than usual Dark Red light on "Error" & "Rebuild".

In the documentation for a different model, Red-Blue flashing means the target HDD for rebuilding, & Purple-Blue flashing means the Source HDD, however, I never selected to rebuild the Array, so does this do it automatically?

Also, why did it suddenly decide to rebuild? - beforehand, I lost access to the drive, so I shut the array down, when rebooting, this happens. I currently don't have access to the drives, or at least, Photoshop doesn't.

tl;dr Is my HDD array actually rebuilding?, why is it rebuilding with no sign of errors or hardware failure?, & preferably, how long will it take to rebuild a 6TB RAID 5 array (4x2TB)?

Edit: It seems whilst uploading that picture to Imgur, Firefox had access to my files, although they may have just been Indexed or whatever by Firefox, seeing as there was no load time.

Edit: Another question: I'm currently refraining from accessing those files - unfortunately, that is quite boring as all of my media is stored on there. Would it be okay to attempt to access those drives whilst it is (Either "Undergoing an Error" or "Rebuilding")?

Reconstruction of current status below: