n999th

I am certain that what you are trying to do is illegal,it's the organization's laptop and not your friend's.
You have to get their permission since it's their laptop.

At this point you could try these:
Swapping the HDD SATA cable. Swapping the SATA power connector. Swapping the SATA port used on the motherboard. Running just the OS hard drive. Swapping to an alternate/new OS hard drive. Trying different RAM if you are still worried about the MemTest86 not starting issue. Other than that I don't have many other idea other than some type of other hardware issue.

Allot of these health checkers are not as accurate as the let on.
 
I have had so many "healthy HDD/SSD's" just drop out with no warning (with S.M.A.R.T. and every other health checker I have tried saying everything is good as the world corrupts around them). But at this point you should check your RAM first not being able to boot MemTest86 is worrying.
 
Check ram first and if that is OK move to swapping drives if you got spares.
 
Edit; if the SATA controller was truly shot every hard drive would be a corruption festival.

Sounds like the exact same thing that happened to my HyperX 5K SSD before its controller kicked the bucket.
 
I would swap the drive, sounds like it's writing garbage and or the NAND is on its last legs; things don't corrupt like that at random.
 
Additionally that SATA cable or controller could be at fault.
 
I have no idea on what is causing the random 500MB partition tho.