Ok, I'll assume that you honestly believe that a hard drive slows down over time to a fraction of its original speed and give you a quick run-down of how it really works.
A hard drive will work continuously short of failure. If you have a hard drive and use it for four years, assuming that you don't have a failing drive it will behave within a very small percentage of how it did when it was new. Hard drives don't use the same cheap bearings that are found in fans. Fans often use cheap sleeve or ball bearings. Hard drives use high quality, hardened ball bearings in a sealed bearing enclosure or (more recently) fluid bearings...which involves no metal contact whatsoever. These fluid bearings last virtually forever and are extremely low friction, as it's just a thin layer of liquid instead of a metal ball. Hard drive bearings therefore last a very long time, as can be seen by the fact that there are still hard drives from the early 90's still going just fine.
When a hard drive starts to whine, it is usually the start of a mechanical breakdown. Typically, a drive that is whining will fail within two weeks. Some times they continue to go much longer, but may exhibit electrical or mechanical failings. Continuing to use a drive in this state is a sign of stupidity in the user. It should be replaced immediately.
When a hard drive starts to click a lot, it signifies one of two things. Very often, it's a sign that your power connection(s) between the power supply and the hard drive itself are poor/loose. Remaking all connections, buying higher quality power splitters, or changing connectors will more often than not fix your problem if you catch it early enough. If you wait too long, it commonly damages the electronics on the hard drive and can cause all sorts of problems from slowness due to damage of the HDD's controller, refusal to pass POST, or a complete stop of functionality of the drive. Electrical damage of this type can take any period of time to manifest itself, but it is usually apparent that something has changed within a couple weeks. The second thing is unrelated to the power, but an otherwise instigated failing of the onboard electronics. The aftermath is similar to the first situation. Unless you are running a fault tolerant RAID solution, you should continue to watch a drive that has gone through a clicking spell very closely and replace it if it continues to do so regardless of what you do.
However, a properly installed and maintained hard drive will happily perform like new for years and years to come.
Properly cool them, make sure you have a solid power supply, use only high quality power splitters, don't allow lots of vibration to effect your drive, keep them away from large magnetic forces, and try to balance the power load of your hard drives. If you do so, they will serve you well for three, five, even six or more years on average. They won't lose speed unless they are damaged, and that is generally the user's fault. Even if it is the manufacturer's fault, continuing to use a drive in that state for anything requiring speed or any degree of reliability is an ignorant decision any way you slice it.
I've got 13 drives in my main system and many more in others. I've seen a lot of them fail in years past and know what causes it. I've run single drives and I've run large RAID arrays of every type. I'm also a judge for a national-level hard drive troubleshooting competition every year. What I'm trying to get at here is that I know what I'm talking about. A properly cared for hard drive is as solid four years down the road as it was when it was new. I also know what improper care can do to drives. That's why I'm posting. Believe it or not...it doesn't make it any less true. I'm also getting sick of giving people advice and information only to be ignored. Later down the road, people usually realize what I was saying and blame me for not telling them.
Do what you want...I'm done
trying to help.
