Do these drives have a separate power supply or are they bus-powered, through connection of the USB cable to the computer? In either case, the circuit board in the enclosure may be at fault, and the hard drive itself may be functional. If you can't get them to start up, you can always open the enclosures. With some brand external drives, the enclosure can be opened by removing screws. With other brands, the enclosure is plastic and snaps together. With this type, you may break some catches securing the upper and lower housings together when attempting to open it. A web search for disassembly of your particular brand drive may provide a web page or YouTube video showing how to open the enclosure. Once you've removed the bare drive from the enclosure, you can connect one of these simple adapters to the back of it (has a separate power supply when needed) and connect the USB cable to the computer. If the drive is still good, you can find replacement enclosures at computer stores. If the drives are bad, you can open up the hard drive itself (usually requires one or two different-sized, small TORX screwdrivers) and remove the platters.