One of the idiosyncracies of Apple’s computers which I find frustrating is the lack of a disk activity light. I often want to know whether a disk is being accessed—usually if I’m not sure whether an action I’ve initiated is actually happening or not—and I don’t like not having that capacity.
Sometimes even Windows machines may not play ball in this regard: the LED light’s defective, the motherboard doesn’t support the function, or your case is situated so that you just can’t see the light.
If you’re a Windows user and resolving this will scratch your itch, TrayStatus from the nice people at Binary Rescue will fill your heart with joy. Apple Mac users can try MenuMeters.
A lightweight free (donationware) utility, TrayStatus sits in your Windows tray and displays icons for CapsLock, NumLock, disk activity, and a number of other keyboard status notifications if you require them.
You can display all of them or, as I do, just the one. The disk activity icon displays green for read and red for write.