External hard drive doesn't mount.

It's a 4TB Transcend used for Time Machine backup. This worked fine for 2 years but has started playing up and now won't mount. It doesn't even appear in Disk Utility. I've tried different USB ports. Also tried unplugging iMac for 15 seconds. I've seen advice to reset the NVRAM but not sure if this is the right action. It's been affecting startup for a while. If Transcend is plugged in it would take 2 or 3 minutes to start up often but this varied. It has also affected powering down. I've had to hold the start button in a couple of times to turn it off and sometimes the Transcend icon would just disappear from the screen. Now it won't even show up at all, not even in Disk Utility. The blue light is lit normally on it and if I pull the lead out a warning box comes up saying I didn't eject it properly which is very odd. It's like it's there but not there. Perhaps it's corrupt? Since I've stopped using Transcend and left it unplugged the iMac starts up and powers off normally and a second WD hard disc mounts normally. I was thinking of reformatting it but can't if it won't mount. Many thanks for reading.