I find the desktops usually boot from the lowest number SATA port or by the boot order in the bios. Be sure this drive is on a higher number then re-test. If not, time to look at your boot order or try the motherboard's BIOS F9 or what key to select the boot drive.
Later after you sort the booting order you can get access to the files and wipe it out.
so i just installed a 750Gb hard drive into my tower which i pulled out of my labtop, i already have windows 10 installed on the tower, and i didnt wipe windows 8.1 bofore dismantling my laptop. so whenever i try to boot my tower with both my 250 ssd and the 750 hdd windows 8.1 takes over and i cant do anything because windows 8.1 cant recognise my mouse, i want to kepp the filed on the hdd and just use it as a soreage disk if possible, if not how do i wipe it clean?