Hard disks have become so huge that less than $200 will now buy you a 500GB external drive in a store. Still, flash drives are catching up in size whilst providing far better access speeds and power usage. (Dell are charging $800 for a 32GB Flash Drive for my laptop – enough to be usable)
Today, major SQL Server installations are unlikely to use flash drives, but the common belief is that this will change in the near future.
But before you start planning for these great 10x performance gains, you should spend some time to understand the underlying differences, and see that it is in fact the nature of your data reads and writes that will affect performance. It really isn’t as uniform as you might think, and it is worth to read the following paper on the subject:
http://www.cs.cmu.edu/~damon2007/pdf/graefe07fiveminrule.pdf
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Update: Spoke too soon.. the flash drive for database systems is just around the corner
Salesforce & MuleSoft Blog 