Many, if not most, older SCSI drives had either provision for attachment of a remote LED indicator, or an onboard LED. This is still true of drives intended for operation in servers, where the ability to 'hot-swap' is important. SCA drives often have light-pipes built into their cradles so that the LED activity is visible outside their enclosures. Most often the LED was/is an indicator of activity, meaning data-writing or -reading rather than just spinning. Their MLBs also have diagnostic points for use during testing after manufacture, and (here I speculate) it is quite possible that one of the LEDs on your drive is for that task.

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