I built a brand new system except for the HDD. I'm not sure how old it is I'd say around 3 years. I read that HDD doesn't affect gaming performance at all?
In a perfect world all game assets would be loaded from the disk into memory. However, many games are still designed around consoles which are 8 years old and have a very limited amount of system and video memory. To make up for this they often stream assets (mostly textures) from the BR-ROM or DVD-ROM, sometimes in combination with the HDD.
This means that HDD performance can have an adverse impact on poorly ported games. The performance hit is often more noticeable on PCs as the game developers often ship higher resolution textures and higher polygon models with the PC versions, but do not make significant improvements to the underlying asset handling.
Some commercial game engines such as Unreal Engine and CryEngine have top notch asset handling, but many in house engines do not.