http://www.disk-partition.com/kb/misconception-hdd-pt1.html
Diagnosis of Common Misconceptions on Hard Disk Drive (Part 1)
1. Frequent disk formatting hard drive will decrease its longevity
The bottom line is: format a hard drive will not shorten its lifespan. Most people tend to accept a false theory that frequent disk formatting will decrease the longevity of hard drive. Formatting is not the cause of HDD failure. The slider (read/write head) does not touch the platters in the formatting process. Thus there’s no chance of physical damage on the HDD during formatting. On the other hand, shaking a hard drive when it’s still working is the real cause in many HDD failure cases. All in all, formatting has nothing to do with the physical damage on HDD. You may even format a hard drive 20 times a day and it wouldn’t shorten its lifespan.