The reason that I never followed up for this problem was that, a few days after I posted the original message, I found that upon turning my computer on in the morning the problem had apparently resolved itself. The internal HDD had reconnected itself, so I assumed that whatever had happened was just a temporary glitch, so I moved on from there.
However, yesterday I attempted to install a program of somewhere between 50-70 GB on the same HDD, and it disconnected again. It occurred in the very late evening, so I simply shut my computer off to see if it would reconnect when I turned it on in the morning. Upon doing so, I saw that the HDD had indeed reconnected itself.
I had made no changes to the hardware of my computer since then, and I have been able to put small files on the HDD without it disconnecting. However, I have been reluctant to use the drive due to the problem resurfacing.
Below is a screenshot of how my Disk Manager appears whenever the HDD is registering as being connected.