If I add an extra hard drive in my computer.

[Siegra]

Will I have to Reinstall the operating system and what will happen after installing? Will I just be able to casually log back on?

 

[Justiceinacan]

Yes, because you can still boot off the original drive. If you are adding an extra drive to SUBSTITUTE for the original, then you might need to get a new windows key and perform a fresh install...

 

[Delroy Monjo]

Just install the HDD on a spare SATA port and log in as usual. The existing OS should find it. You may have to format it. It should show up in your BIOS page as well under Storage or something like that.

 

[D_Know_WD]

Hi there Siegra,

I will provide a couple of tutorials.

As the other guys have already mentioned, you just need to install the drive in your computer. You will need a SATA as well as a power cable. Once you do that, you will need to initialize -> partition & format the drive.

How to initialize: http://products.wdc.com/support/kb.ashx?id=Drb6x1
How to partition and format: http://products.wdc.com/support/kb.ashx?id=9k5Tqf

Well, this is the procedure, if we assume that you want your new drive, to be a secondary one.
If you want it to be your OS drive, you will either need to perform a clean OS installation, or clone your current drive onto the new one. This would create exactly the same copy. Most of the HDD manufacturers have their own cloning tools. Some third party ones would do the job as well.

Let us know in case you have some more questions,
D_Know_WD