The hard disk access light on your computer signifies when your computer is reading / writing data to your hard drive (s). This can happen for a number of reasons, but is mainly down to your programs using the data on your hard drive. You should expect at least some activity from this Windows process, but if you're seeing the hard disk drive light constantly on, then it's a sign that something's wrong, and here's how to fix it …
If the hard drive light is constantly flashing, there are several causes that could be behind it. If you fix these causes, your PC should stop accessing the hard drive, making your system faster and more reliable.
# 1 – Check For High-Memory Programs – The first thing you need to do is to stop any programs running that is draining the memory of your system. Each time you use your PC, it has to load up a series of programs but it's often the case that many of these will malfunction & try and access a lot of data that's inside your system. This will explain the constant access to the hard drive in most cases, and in order to fix this problem, you should first press CTRL + ALT + DEL to bring up the "task manager". When you have this open, click on the "Processes" tab and then click on the "memory" column to organize the programs by memory usage. You should then end any of the programs that are using too much memory, as they might be malfunctioning.
# 2 – Defragment The Hard Disk – After you've done step 1, you should then try and 'defragment' the hard drive. This is a process which is in-built inside Windows, and works to put all your hard drive's files into the right order again. It's often the case that Windows will "split" (fragment) many files and save them in different locations on your system, which can cause the constant access of your hard drive. To fix this, click on Start> All Programs> Accessories> System Tools> Disk Defragmenter. This is a Windows tool which will defragment your HDDs for you.
# 3 Clean Out Junk Files – When you've defragmented your hard disks, you should then clean out all of the "junk files" from them. To do this, you just need to look through all your download folders and delete any files that you do not want any more by selecting them and pressing "SHIFT + DELETE". This will permanently delete the files and free-up hard drive space. Alternately, you could get some software tools to do this automatically.
# 4 Clean The Registry – The 'registry' is also a big cause of the constant hard drive access you're seeing. This is a database that stores all the settings and options for Windows, and is best described as being the "central memory" for your computer. The registry database stores settings for all your software & your system – including details such as your latest emails and even your desktop background. Unfortunately, many of these settings become corrupted which leads your computer to have to constantly read the files, causing the constant hard disk access. To fix this problem, you should download a 'registry cleaner' which will scan through the registry database and remove any of the damaged or corrupted files that are inside there.
Source by Greg Harry Barker