USB Portable HD Only Mounting Occasionally

So I feel like this is going to be a really stupid question, or at least have a simple result, but here goes:

I have a brand new Pi3 and am running LibreELEC 8.0.2 and Raspbian.

I got libreELEC set up with my hard drive just fine.

I rebooted into Raspbian to see how it was and switched back to LibreELEC .

But the HD wouldn't mount. Or at least didn't on it's own. I rebooted, again hoping to fix it and nothing. Since then I've gotten it to mount twice, but it never holds through a reboot.

The only way I've gotten it to mount again is by plugging it into my laptop and manually ejecting it, then booting the pi up with it plugged in. But even then it's not consistent. Probably about 1 in 8 times.

I've done my best searching the web, have tried ssh-ing in to manually mount it, label the drive, and anything else I saw online that I thought might work.

I'm new to linux based systems, so I probably did those things wrong... But I can't figure out how to make this work.

Any help yall could offer this confused noob would be very appreciated.

Thanks.

(I feel like it has something to do with not being fully ejected, but I can't figure out how that could be if every time it comes out I have either properly powered off or ejected it...)

That may be so..... I got a "2.1 A" supply, but it was pretty cheap... maybe not living up to its specs. It is powering both the Hard Drive and the PI. I guess I figured since it was a pretty small hard drive it might be enough, but I guess maybe not?

Thanks I'll give that a try.

It turned out to be my micro usb cable. I didn't suspect anything cause it felt pretty thick and decent. But I went and got a new one, and its working like a charm. Thanks a ton for pointing me in the right direction though.