|Yup they make them. Here is one such switch - http://www.cyberresearch.com/store/pc-accessories-computer-parts/computer-accessories-pc-peripherals/hubs-switches-switch-boxes/DSW_54KA_4278.2.htm|
Do a Google search for hard drive swithces and you will be presented with many options.
As far as building one yourself, I wouldn't do it. leave it to the pros who know what they are doing. Not to question your skills but you would have no way of testing it to make sure the data transfer rate is up to spe3ed or if you are losing/corrupting data.