Life Expectancy of an HDD?

Hello, so I am building a new PC and I am reusing an HDD from my old computer, the HDD itself is about 4 years old, and it's still running fine, but I'm wondering, will it last much longer? Should I buy another HDD just incase and transfer all the data from the old one? Or should it be fine for another couple years?

The time it has left depends on how much time it was actually in use, if it was in an office PC that was on 8-10 hours a day it would be close to the end of it's life cycle, if it was in a PC that was only used a couple hours each day, it may still have some time left. 

I have here on my desk 2 discs from client's PC that started giving errors and I had to replace, both are from 2008.  I would recommend that you replace the disc before it's 5 years old.

We kept the computer on 12 hours a day mostly, but we weren't always using the computer

No way of knowing how long it will last.

This might be helpful.

http://en.wikipedia.org/wiki/Hard_disk_drive_failure

And if nothing important/irriplacable is stored on drive, I wouldn't worry and just replace when old one fails or you want a faster/bigger one.

The only thing important is windows 7, which I lost my disc/cd key for, but I think I'll keep this drive for a few months, then transfer the data to a new drive later

It depends on the quality the environment luck and much more so predicting is really hard. If you fear that your drive might go kaput you should look a the SMART values.

My friend had his old hard drive for seven years and the computer was on 24/7. Sadly he had to throw it away when he built a new pc since it wasn´t compatible with new motherboard.

Ive had HDDs that were over ten-15 years old.(like 20GB ones) Used regularly in a school environment with frequent formatting and rough handling by new students learning to build PCs.

There really is no way to tell for sure. 

Again, because people still say that you should do nothing. Look at the SMART values. They tell you if the drive will fail, soon. Of cause it can't know If you throw it around or a lightning hits it but for normal fading this works great.

Agreed. If you see that it detects any bad, or potentially bad or unrecoverable sectors, it's not going to last much longer.

Run disk checks more frequently on older hard drives to be safe. 

 

I've had some disks last a couple years and I've had some last a dozen years. It just depends on it's lifestyle. Moderate temperatures help, average/low use helps, etc. Find a SMART tool and check it out. I used to use (mind goes blank) and it would tell me how many hours/days/years the drive has accumulated. If you look up the manufacturing specs then you'll be able to tell what the drive was rated at. If you're getting close to that on any of the numbers, then I'd consider it a spare/cache drive.

More importantly, as mentioned above, do a deep scan say once a year (being minimalistic here) or so for bad sectors.