Hard drives are the component of your computer that stores your files as well as what your operating system is installed on. If you didn't use a hard drive you would not be able to run your computer unless you temporarily booted it up from something like a CD or USB flash drive.
In order for a hard drive to be recognized by an operating system such as Windows, the computer needs to know that it exists, is connected properly, what size it is and so on. For IDE devices such as hard drives and CDROMs you need to make sure the jumpers are set correctly or else the computer won't recognize the drive and you won't be able to use it in your system.
Although IDE hard drives are starting to be replaced by SATA drives which offer faster data transfer rates there are still many computers out there that use IDE drives. If you ever have ever had to replace an IDE hard drive or CD/DVD drive for that matter you may have noticed that there are jumper settings that have to be set to make the computer recognize the drive correctly. A standard IDE cable will have two connectors on it for the 2 IDE devices. Some cables may only come with one though. The slave drive does not rely on the master drive for its operation or vice versa. It’s just used so the computer knows which drive to send data to.
Most computers come with two IDE controller ports on the motherboard that will support two drives each for a total of 4 drives in the computer. The only time you really need to worry about which drive is master and which is slave is when you are setting up your operating system drive on the primary IDE controller port. You should set the OS drive as master and connect it to the primary IDE port so it boots the computer correctly.
When connecting more than one hard drive or other IDE device to a computer on the same IDE controller, you have to assign one as the primary or master and one as the secondary or slave using jumpers.
The placement of the jumpers will vary on the hard drive. The drive should have a diagram on the back or top of the drive indicating what position to put the jumpers for what setting.
Cable Select (CS) settings were designed to make it easier to connect hard drives by making it where you didn’t have worry about the Master or Slave jumper settings. You simply set the jumper to Cable Select on both drives and depending on where you connected them to the cable the computer would know which is Master and which is Slave. If you are going to use Cable Select you need a special 40 conductor IDE cable that would determine master and slave connections. Most IDE cables you buy today should be of this type.
If you set both drives on the same cable to master or slave then the computer most likely won't recognize the drives in the BIOS and therefore you won't be able to boot the computer with the drive or install your operating system on it. So always check your settings when installing a new drive. If you are replacing an existing drive simply set the jumper to match the setting on the old drive.