Recently my hard disk corrupted and my HDD reported as being 100% in use all the time. This slowed my computer to a crawl and became totally unusable at times. I fixed it by changing it with an SSD. However:
I noticed the Task Manager looked something like this. Notice disk usage is at 100% even when applications are using 0.1Mbps. 
However, I've noticed while reading and writing anything that speeds easily cross 100Mbps but the usage stays at 70-80%.
So my question what value is this a % of?