Hard drives are the biggest bottleneck in your computer... why slow it down even more using compression? Buy a bigger hard drive... 500GB drives are really cheap... probably even cheaper than the Raptor you're considering.
Compression slows down your computer... no matter how fast it is, you WILL notice a difference. This is because compressed files have to be decompressed on the fly in order to be read. I have never liked the option (even back in the DOS days) and big hard drives are just too cheap nowadays for it to be even necessary.
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