It is also important to note that heat can drastically reduce the lifespan of hard drives.
The following is an excerpt from:
http://www.pcworld.com/article/id,78048-page,1/article.html
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"Michael Beary, General Manager of ESS Data Recovery in Carbondale, Illinois, explains it simply: "Heat causes the metal oxide coating on the hard drive platters to expand. This affects track alignment and weakens the magnetic strength of each bit over time, eventually leading to hard disk failure." In fact, Beary says, heat is the number one cause of hard drive failure. And it doesn't take a catastrophic house fire to toast a PC's hard drive or other component--even a heavy session of multitasking could do it. "As hard drive technology packs more and more bits into each square inch of platter space," Beary says, "hard disks become more and more sensitive to heat. Since most PCs don't come with hard drive cooling units, the best way to preserve the life of your hard drive is to install a fan."
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In a DVR, the hard drive is constantly transferring data with the drive.