[SOLVED: See my comment below]

I'm using a Lenovo laptop with 2 drives: an SSD and a HDD. The OS is obviously on the SSD.

Specs:

SSD: SanDisk SSD Plus 120GB

HDD: Seagate st500lt012 (500 GB)

CPU: Intel i5 4200m

RAM: 8 GB

I can sometimes boot into windows 10 anniversary edition, but other times, the circle spins infinitely, and Windows never boots. Other times, I see the automatic repair text appear and then disappear. Still other times, the automatic repair text appears, and then it says that Windows Diagnostics is running (I cant really remember what the text is). Still, nothing comes of it and I am unable to boot. The reason why I titled it "occasional boot failure" is because restarting and trying to boot over 30 times ends up with the computer actually booting properly once.

I am able to boot into Ubuntu live edition.

My SSD is 6 months old, and the HDD is 3 years old. Most diagnostics say the HDD is in good health too, but there are apparently some questionable statistics about it. (I did not perform these checks, but I was told that the drive looked mostly fine, but that it could have gone bad.

I've reinstalled Windows, doing it perfectly. Nothing connected, only my SSD connected, using Windows Media Creation Tool. I reformatted the SSD and did a clean reinstall. I then backed up the files on the HDD, and reformated that too, and then put the files back on it. I'm now on windows build 1607, version 14393.447.

I also occasionally have seen file explorer hang and stop responding when trying to access files on my HDD. I don't know why this happens, but it may be related.

Also, IDK why both my drives show up in the bios as "Legacy" instead of "UEFI". Theyre both connected via SATA ports. My HDD is actually in an optical disk caddy, and so it uses the optical disk SATA interface. (This is probably not the problem, as I've used this setup on Windows 8.1 since March and I've never had this problem before. I upgraded to Windows 10 in September-ish and I dont believe I had this problem then. I reinstalled Windows 10 since then, so it's probably not causing my problem.)

As long as my laptop is on, it works pretty much perfectly (except for that file explorer on occasion stops working). Checkdisk doesnt say anything is wrong. It took ~2 hours to run this on a 500 GB HDD connected via USB. Idk if this is an indication of anything.

I had three such bootloops in the last 48 hours. Two were fixed by booting and shutting down enough times. The 3rd time, however, I physically removed the HDD from my laptop, and it booted instantly. This leads me to believe that my HDD is dying, but no test supports this hypothesis.

If anyone is able to help me with a genuine solution, please comment below.

Obviously, reinstalling windows or refreshing windows will not help. Do not suggest this. My last windows reinstallation of windows was literally 48 hours ago.