To clarify, I'm referring to entire disks disappearing, not just partitions. The PC in question is an HP Pavilion 2325DX. Both drives are variants of HGST. One is in the primary slot, the other is in an optical caddy. Partition style is GPT/UEFI. Each disk has 1 EFI partition (per disk). In Windows, the drive will be there sometimes, but sometimes gone after reboot. It only happens on the secondary drive, not the one Windows is installed on. It also seems to make no difference which slot each drive is in. My Linux install sees both drives perfectly all the time. It seems to be completely random. It doesn't happen often, just enough to annoy, and is hard to get it back again. This has perplexed me for over 2 months.
What I've tried:
1. Called HP, won't help unless I pay a hefty fee and won't give any info, PC is a gift and no longer warrantied.
2. Called HGST, they say they've never heard of such an issue, no firmware updates available, rep never called me back.
3. Swapped the drives into the other slot, issue went away for awhile then returned.
4. Installed all Windows Updates and latest drivers.
5. Tried Win 7/10, same issue.
6. Ran SMART/diagnostics tests, drives are relatively new and came up clean.
7. Put both disks into an external caddy, they read fine from Windows/Linux, and from friends' PCs.
8. Tryed varying partitions sizes of less than/more than 1TB, seems to have no effect.
9. Downgrading/Updating BIOS, no effect
I'm thoroughly confused in regards to what could be causing the issue and don't know where to start looking, new optical caddy is cheapest option but not guaranteed to work. PC is otherwise satisfactory in all other respects.
Thanks in advance!