How do multiple hard drives speed up PC performance?

"Because sometimes your computer is performing more than one disk activity at once.

Example: you're loading ___MB of game assets from disk #1 while Windows does its pagefile activity on disk #2.

Another reason for a hybrid ""fast small disk + slower bigger cheaper disk"" strategy is cost. Ideally, you'd have 5TB of fast solid-state storage and no spinning disks. That's not cost effective unless you're the king of an oil kingdom, but most demanding users can probably afford a $250 80GB Intel SSD to speed up OS and key software applications in addition to the 7200RPM drive they were going to buy anyway."

"Because you can read from two different sources at once.

Your HDD can only read one thing at any one time, and to read another it has to move the read head, two drives can have potentially double the throughput (Two identical drives WILL have double throughput in RAID 0)"