[I don't know why, but every time I submit a Reply, I get an Error Occurred. But the Reply goes through anyway.]
Boot into the BIOS and see if the all the disks show there. If not, find out why.
You're right gderf. In BIOS, two SATA ports show "none". And with SATA 0-4, it's the SATA2 and SATA3 that are blank. I'll do some investigation and see if there's anything obvious, like a bad connector or something.
One other thing I was wondering ... since three data disks were unionfs before, will that make any difference about seeing data? I have the Paragon Software on my PC that allows me to look at the drives in Windows File Explorer. Shouldn't the content of files be seen in there somewhere?