Here are a couple of data recovery enquiries I received about the feasibility of getting the files back from Seagate and Drevo hard drives that are no longer showing up.
My Drevo X1 Pro SATA III 256gb SSD hard drive has failed. I'd had problems with my MacBook Pro previously, where it couldn't connect to my old HDD. Replaced it with an SSD, still couldn't connect. Replaced the hard drive cable, which solved the problem. All working fine until one day out of the blue I had the same issue again. The MacBook couldn't find the SSD or boot up etc. Initially thought it was the same problem, ordered a new hard drive cable, but that didn't fix the problem. Took it to the campus IT shop, they ran some tests, came to the conclusion that the hard drive was faulty, as he couldn't connect to it from his systems either. His comments on the invoice were as follows: "Cannot pick up SSD hard disk. Tested hard disk in a another PC – also not picked up either in bios or macdrive diagnostic software. Tested the cable with known good Mac hard disk – ok. Conclusion – faulty Drevo SSD hard disk." He also said that it looked like whatever connectors there were in there, that they could be faulty.
My comment: Until I've taken a good look at each drive to discover what's wrong, I'm not able to say if I can recover the files. From the symptoms above, I', hopeful that I can get the drives showing up again and retrieve the files, but I really do need to diagnose them first