This will probably sound like a really noob question but what should a HDD's usage be sitting at while installing a game on it? I randomly looked the other day and noticed that mine was at sitting 99-100% while installing a game, is this normal ?

so you want your drive to throttle?

Was thinking this could be working like my CPU; fluctuating between certain percentage

Cpu's fluctuate on the load demand...
ssd's could in theory fluctuate as well for small files but under a heavy load you'd want to ideally use all available power to complete the process if possible just as you would for a cpu load.

Awesome, I was kinda worried that this could be signs of defective HDD.

Thank you!