TCQ stands for the Tagged Command Queuing technology built into certain PATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. TCQ is almost identical in function to Native Command Queuing (NCQ) used by SATA drives.
Before TCQ, an operating system was only able to send one request at a time. In order to boost performance, it had to decide the order of the requests based on its own, possibly incorrect, idea of what the hard drive was doing. With TCQ, the drive can make its own decisions about how to order the requests (and in turn relieve the operating system from having to do so). The result is that TCQ can improve the overall performance of a hard drive.
