Q I want to buy a pair of new hard disks for my old-style Mac Pro, which supports SATA-300 but not SATA-600. However the ideal 2 TB drives that I would like are listed as SATA-600. Would they still work, and do you have any other advice?
A There is no fundamental difference with respect to compatibility between SATA 300 and 600: they use the same connectors and signal specifications, and differ only in performance. Put a 600 MB/s drive in a 300 MB/s system such as yours, and you will only get 300 MB/s performance, as you will from 300 MB/s drives in either host.
However it is always worth purchasing from a dealer who will exchange the drives if they do not work, as some models can prove pernickety.
If buying more than one drive at a time, try to purchase from different dealers, or different models, so that you do not end up with two or more from the same manufacturing batch. Studies have shown that drives from the same batch tend to fail at about the same time, which could prove disastrous if they were both installed in the same Mac.
Updated from the original, which was first published in MacUser volume 27 issue 23, 2011.