Contrasting the advantages of magnetic disk storage and main memory is a bit like comparing apples and oranges. Magnetic disk storage is used in mechanical hard drives to keep track of permanent information. Main memory, on the other hand, is used by RAM chips to temporarily store the data the computer needs to calculate and track the programs it runs. Computers use both, because each has its own advantages.
The biggest advantage of magnetic disk storage over main memory is persistence of data. The RAM chips that house your main memory contain huge numbers of tiny electric switches, each storing a bit of information. When the power goes off, all of that information disappears forever. Your hard drive, on the other hand, has a magnetic platter covered with tiny zones of charged magnetic fields. Those magnetic fields stay there, storing your information even when you aren't actively using your computer.
One of the greatest things about magnetic disk storage is its ability to fit a tremendous amount of information into a small amount of space. Hard drives stay the same size but are able to fit more and more information onto magnetic platters. High-end hard-drive storage is measured in terabytes -- thousands of gigabytes of information. Memory sticks cannot store nearly as much data.
Since the beginning of computing, hard drives have come down in price, and they continue to do so. Main memory also has become a lot cheaper, but its storage limitations make a true comparison difficult in this area. A RAM stick often holds less than 1 percent of the data capacity of a hard drive with the same price.
- hard drive interior image by Curtis Sorrentino from Fotolia.com