2 SATA drives stopped getting along

[cecinit]

I long ago upgraded the SATA drives on this old workhorse.  It boots & runs Windows from a 1 TB SSD (I upgrade these every few years as size and cost equation changes).

I have an 8 TB and 10 TB WD hard drives in the cage.   These two got along ok for about 6 mos.  Prior to that, there was a 6TB drive in place of the 10 TB (I've been doing the same kind of laddering as with SSD's).  I've been doing various kinds of mirroring between the 2 disks, lately using just robocopy.

A few months ago, the 10TB just disappeared.  After that, I found it was recognized at bootup but not mounted.  I thought it had just failed (infant mortality, a bit late) but when I unplugged the 8 TB, the system mounted it on reboot.  This 10TB drive tests fine, and no bad disk stats are shown.  After some recent experiments, I have concluded that both disks are ok as far as I can tell, but they no longer like to be on the same SATA bus.  Why is this?  What is the problem?  I am at a loss to figure it out.

The SSD is (fortunately) unaffected by this as far as I can tell.

Any ideas on what the problem is, & how to fix?  Thanks, ==mwh

[erico]

part of the problem is that you are running an unsupported OS on an older generation desktop.

If you reinstall the 6TB drive that you stated that you previously had installed and it works as expected, that will point to the 10 TB HD as the culprit.

[cecinit]

"If you reinstall the 6TB drive that you stated that you previously had installed and it works as expected, that will point to the 10 TB HD as the culprit."

Mmm... actually what one would learn from this is that either one of the disks doesn't like each other.   One would still not know which one is the culprit, or more importantly, why.   IE it doesn't help - I'd be in about the same place.

I'll consider doing mix-and-match at some point but that level of system mechanical work takes time, especially downtime. I'd like to have more of a theory before doing that work.

Incidentally,

"part of the problem is that you are running an unsupported OS on an older generation desktop."

That's a so-what at this point, since Windows 7 is no longer supported & the vendor doesn't support the desktop anymore.   Frankly there's no evidence that either point is part of the problem.  It wasn't before.  Something changed, but what?