What makes a hard drive sluggish

Your hard disk accumulates a lot of TMP files, work space areas, backup files and downloads for updates and Internet caching during the course of a day.  Most of this is cleaned up by applications and the operating system from time to time, but there are always exceptions or files that tend to remain over time even though they are no longer useful and just 'taking up space.'

Having to weed through these files, especially if they are fragmented across many sectors of hard disk space can eat up a lot of passes and cause your already slow 16-bit data access to wait while files are located and read.  Part of the problem is that your hard disk unlike the human eye can't 'scan' the entire disk surface area and then find a location, zoom in and read what's there. 

The system works in a sequential manner, and the circular plates spin to allow static heads to read data much like a phonograph plays a record.  If the index says a file starts at sector 4 then the head moves to that location, starts reading and as the disk spins finds the starting sequence that say "I'm the file" and begins to read.  In an ideal scenario the series of data stored is all sequential and it will read data after data until the entire file is loaded into memory … in the real world this is almost never true even after a defrag, because reserved space, damaged sectors etc. cause some fragmentation even in an optimized disk. 

Having found the start of the file and read to its end point, the 'next' file piece location is then read, and the head moves to that sector, and waits until the spinning platter again delivers the start of the next segment of data … you can see how this can be time consuming right? Especially if the disk is spinning slower then normal because of age, heat or low power conditions or if the disk has lots of damaged sectors forcing lots of movement.

By far fragmented files as mentioned above is the cause for most disk access problems, but damaged disks is common as they get older as well, and while built-in error checking and correction normally prevents you from even noticing that your hard disk is having problems it can't overcome the need to 're-read' data that failed a read test or didn't appear to have the correct number of bits etc. – if you are still seeing slow performance after regular maintenance and defrag you need to run a ScanDisk or other utility to check the condition of your surface platters. 

If there are bad sectors showing consider replacing your hard drive, but definitely make certain you have backups!  With a hard drive it's not a matter of if but when it will fail – and since your entire system resides in those intangible bits of magnetic fluff having a second copy handy might just save more then time.  It could save your data!