For testing programs on different OS platforms, its useful to have the ability to boot from different hard drives. In many solutions the data cable is cut and the master / slave signal is crossed via a switch.
The selection between hard drives is done by master / slave jumpers. Remove the jumpers an connect a 2pin connector instead On the connector solder a cable, on the other side of the cable solder a switch. This switch selects the master /slave.
this solution has the advantage, not to cut the hdd cable. Disadvantage, there is no standard jumper assignment.
Sample for 2 equal HDD's (SV6004H)
This type can be set to 32Gb clipping. In my case I don't use the 32Gb clipping, which means jumper 3 - 4 has to be shortened. With the jumpers 1 - 5 resp. 1 - 2 the master / slave is selected
Connection HDD 1 The common pin of the switch is connected to HDD pin1, the NO pin is connected to HDD pin 2 and the NC pin is connected to HDD pin 5.
Connection HDD 2 The common pin of the switch is connected to HDD pin1, the NO pin is connected to HDD pin 5 and the NC pin is connected to HDD pin 2
Some hdd's have both jumpers in vertical direction, in that case find the gnd pins, the opposite pin is the master / slave pin which is selected by the switch.