Hard Disk Monitoring

Hi there,
I have a 8 TB HGST hard drive I've been trying to "stress test" - its a refurbished drive that replaced one that died under warranty. So I've had it do the "read+write test" and "extended self-test" under HD Sentinel. And it done fine - except that after the read+write test is finished, the drive is missing from My Computer. I can still see it in HD Sentinel and Disk Management, although its listed as "unallocated". I do "new simple volume" again, assign it a drive letter, etc., and it works just fine. Then I do read-write test again, and once its finished again its back to unallocated.

I want to make sure there's nothing wrong with the drive before I trust it with data. What's going on here?

Its possible that there's something wrong with the PC - its ten years old and has developed some quirks over time, including the occasional random blue screen, that may indicate problems with the motherboard or other parts.

Sorry, I do not really know which test you mean, as there is no "read+write" test available.

Did you use the "Write+Read" test? As its name suggests (and the description of the test says) it performs a complete overwrite and then read back the written data, to verify both write/read functions of the disk drive. This is generally an excellent stressing method - but it is completely normal and expected that it deletes all stored data from the disk drive. There are warnings displayed before starting this test, exactly to increase attention that the stored data (including all partitions) will be lost.
Then the hard disk drive does not disppear - but in Windows Disk Management it is listed as unallocated and you'd need to create a new partition, assign drive letter and format it (a quick format is completely enough as the drive extensively tested already).