As you probably know, the answer is...it varies. It may be to control spread spectrum clocking, used to reduce EMI, and/or to force the drive to reduce throughput to that of an "old school" host controller that only supports SATA Revision 1.x (up to 1.5Gb/s). If you're wondering why your wiz-bang, high-speed hardware that supports SATA Revision 3.x (up to 6Gb/s) is slower than molasses, check those jumpers my friend; don't let 'em bring you down.

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