External hard drive not recognised by MacBook Pro. Recently my segate 1tb external hard drive has been playing up on my late 2013 MacBook Pro with retina. It has been very inconsistent. sometimes it works fine for a day or two or only 2 minutes, and then other times it doesn't even show up. I have looked in disk utility and I cannot find it there. I tried to run 'first aid' on it in disk utility yesterday but got a message saying something along the lines of "cannot perform first aid because the drive cannot be un-mounted. Thanks in advance for your help!
First off if you notice issues with an external drive like you describe that is the drive trying to tell you that it is failing. Hope you have a backup.

If it is not showing up in Disk Utility you could try rebooting the Mac, with the drive attached, a time or two. Power cycling the drive, unplugging it and plugging it back in, all of which might get it working or might do nothing at all.

If you can get it to mount and there is nothing you need on the drive try re-formatting it. If that fails then you definately have a bad drive instead of just a corrupted one.

If you have a third party disk repair utility (I like DiskWarrior) and it will see the drive run that.

Honestly it sounds like the drive is dying and needs to be replaced, either under warranty or by buying a new one.