2 internal hdd's slow computer?

im planning on buying a internal hdd to add with my existing one but i heard that 2 hdd's means your start time to boot up your computer and to shutdown will be longer and the chances of failure increase in both drives because they are always on when the machines on and are close together and generate double the heat?

 

As long as the drive isn't corrupted it will have no impact on startup times.
Don't worry too much about drive failure, but it's a good idea to put a case fan blowing on the drives. That will definitely increase there longevity.
My mediapc has 5 hard drives in it running 24/7, in 3 years I haven't lost a drive to failure.

My recommendation is to partition your new drive into 2 partitions (30gb partition / and remainder data partition), that way if your original drive fails you can just use your new drive as a boot drive without deleting the data partition.
All my computers have at least 2 drives in them, it lets you to instant backups without hooking up external drives etc.

 

I have four internal HDD, as long as you have a proper PSU, you have nothing to worry about.

 

I don't think it affects booting times or everyday usage, unless you use a RAID mode

 

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As others have noted adding another drive shouldn't cause any problems. Just make sure there's space between the drives to allow for good air flow. There are actually hard drive fan coolers designed to blow air directly at the drive, which would guarantee proper cooling - dirt cheap on eBay.