Additional HD "disappearing"

First off, I apologize if this has been handled before--I tried to search for threads on it--maybe I used the wrong words, maybe it's NOT a duplicate issue, I am not sure... I don't often use forums, I am, I would say, 99% self-taught (so not very well-versed in "proper" terminology for hw)
Anyway, I have 4 HDs installed--my motherboard detects them all in BIOS.
No matter which "flavour" of ubuntu (or any other linux distro, tbh) that I use, this problem occurs--
When I first boot up, all 4 of my HDs USUALLY show up, and are mountable. But, after a (seemingly random) amount of time, one of them suddenly disappears. From GUI, AND lsblk, fdisk, etc.
I have tried using different SATA cables, different SATA ports--everything I can think of--but inevitably the same thing occurs. According to the SMART data, the disk has 3 bad sectors. I have had the HD for a couple years--but when it's seen, it works as it always has.
No RAID arrays (the disk has never been part of a RAID--always used it as plain old .ext4 data storage)
The drive was mounted at /dev/sdd before it disappeared most recently.
I CAN just pull the data off of it and replace it if needed, but I would like to know if it is simply something I overlooked in upgrading to 16.04.
This computer was ruing standard Ubuntu 14.04, and it read the HD every time without the random dropping--so I wonder if it could be a bug, or if my HD may have failed when I moved the computer to the other side of the house (lol--but I KNOW that to be a possibility.)
When I decided to upgrade, I used a Live DvD, full reformat, repartition, and clean install.

Not sure if I missed any possibly relevant info...