Holy cow, I wrote a book!
Back in Windows 95, people would notice that their hard
drive light would blink every few seconds. What's that all about?
Actually, it wasn't the hard drive light after all.
Windows 95 was polling your CD-ROM drive to see whether
you had inserted a new CD. Some computers wired up the
"hard drive light" not to the hard drive but rather to the
SCSI and/or IDE bus. So the light didn't indicate hard drive
activity necessarily. It turned on for any bus activity.
Fortunately, motherboard manufacturers discovered their mistake
and nowadays you don't find any of them that miswire the hard
drive access light.
Or do you?
I keep my computer under my desk so I never see the hard drive
light anyway. I'm just surmising that in the past seven years,
motherboard manufacturers have gotten their act together.