My best attempt at a timeline of how I dug my grave... I had been running fine for months with:
    Apr 17 19:51:40 RatCreek kernel: mdcmd (1): import 0 sdb 64 3907018532 0 ST4000VN006-3CW104_ZW6280MG
    Apr 17 19:51:40 RatCreek kernel: md: import disk0: (sdb) ST4000VN006-3CW104_ZW6280MG size: 3907018532    PARITY 4TB
    Apr 17 19:51:40 RatCreek kernel: mdcmd (2): import 1
    Apr 17 19:51:40 RatCreek kernel: md: import_slot: 1 empty
    Apr 17 19:51:40 RatCreek kernel: mdcmd (3): import 2 sdd 64 3907018532 0 ST4000VN006-3CW104_ZW624CEQ
    Apr 17 19:51:40 RatCreek kernel: md: import disk2: (sdd) ST4000VN006-3CW104_ZW624CEQ size: 3907018532    DATA 4TB
...showing & ignoring "missing disk" where I had removed "data disk Samsung_SSD_870_EVO_500GB_S6PXNS0W524500P (sdd)" and made it Cache.  I failed to persist in finding how to remove that apparently-cosmetic error. 😞
Recently I added a new 8TB ST8000VN004-3CP101_WWZ6KWYX physically to the array on an unused data plug, and selected the new disk in Array Devices' dropdown, displacing ZW6280MG, without RTFM the procedure.  Also at the same time, selected the old ZW6280MG in the dropdown for the "missing disk 1" that had been warning ever since shifting the SSD to cache. Then booted a couple of times and in Maintenance it performed Parity build/copy for ~12 hours onto the new WWZ6KWYX, shown as healthy parity.  Then I rebooted and it showed:
    May  5 13:56:57 RatCreek kernel: mdcmd (1): import 0 sdc 64 7814026532 0 ST8000VN004-3CP101_WWZ6KWYX
    May  5 13:56:57 RatCreek kernel: md: import disk0: (sdc) ST8000VN004-3CP101_WWZ6KWYX size: 7814026532    PARITY (new HDD)
    May  5 13:56:57 RatCreek kernel: md: import_slot: 0 wrong                                                <------------------
    May  5 13:56:57 RatCreek kernel: mdcmd (2): import 1 sdb 64 3907018532 0 ST4000VN006-3CW104_ZW6280MG    OLD PARITY, still in its old physical position/connector
    May  5 13:56:57 RatCreek kernel: md: import disk1: (sdb) ST4000VN006-3CW104_ZW6280MG size: 3907018532 
    May  5 13:56:57 RatCreek kernel: md: import_slot: 1 replaced
    May  5 13:56:57 RatCreek kernel: mdcmd (3): import 2 sde 64 3907018532 0 ST4000VN006-3CW104_ZW624CEQ        
    May  5 13:56:57 RatCreek kernel: md: import disk2: (sde) ST4000VN006-3CW104_ZW624CEQ size: 3907018532     DATA  (no change)

In Array Devices, I changed ZW6280MG (OLD PARITY) (DISK 1) to UNASSIGNED, and rebooted
    May  6 21:31:26 RatCreek kernel: md: unRAID driver 2.9.33 installed
    May  6 21:31:26 RatCreek kernel: mdcmd (1): import 0 sdc 64 7814026532 0 ST8000VN004-3CP101_WWZ6KWYX
    May  6 21:31:26 RatCreek kernel: md: import disk0: (sdc) ST8000VN004-3CP101_WWZ6KWYX size: 7814026532      PARITY (new HDD)
    May  6 21:31:26 RatCreek kernel: md: import_slot: 0 wrong                                                <------------------
    May  6 21:31:26 RatCreek kernel: mdcmd (2): import 1
    May  6 21:31:26 RatCreek kernel: md: import_slot: 1 empty
    May  6 21:31:32 RatCreek kernel: mdcmd (3): import 2 sde 64 3907018532 0 ST4000VN006-3CW104_ZW624CEQ
    May  6 21:31:32 RatCreek kernel: md: import disk2: (sde) ST4000VN006-3CW104_ZW624CEQ size: 3907018532 

I unplugged OLD PARITY ZW6280MG 
    May  6 23:24:47 RatCreek kernel: md: unRAID driver 2.9.33 installed
    May  6 23:24:47 RatCreek emhttpd: Starter key detected, GUID: 0..2 FILE: /boot/config/Starter.key
    May  6 23:24:47 RatCreek kernel: mdcmd (1): import 0 sdc 64 7814026532 0 ST8000VN004-3CP101_WWZ6KWYX
    May  6 23:24:47 RatCreek kernel: md: import disk0: (sdc) ST8000VN004-3CP101_WWZ6KWYX size: 7814026532     PARITY (new HDD)
    May  6 23:24:47 RatCreek kernel: md: import_slot: 0 wrong                                                <------------------
    May  6 23:24:47 RatCreek kernel: mdcmd (2): import 1
    May  6 23:24:47 RatCreek kernel: md: import_slot: 1 empty
    May  6 23:24:47 RatCreek kernel: mdcmd (3): import 2 sdb 64 3907018532 0 ST4000VN006-3CW104_ZW624CEQ
    May  6 23:24:47 RatCreek kernel: md: import disk2: (sdb) ST4000VN006-3CW104_ZW624CEQ size: 3907018532 
    
I plugged  OLD PARITY ZW6280MG back in to its orginal plug; it doesn't show in syslog.
    May  7 13:23:20 RatCreek kernel: mdcmd (1): import 0 sdc 64 7814026532 0 ST8000VN004-3CP101_WWZ6KWYX
    May  7 13:23:20 RatCreek kernel: md: import disk0: (sdc) ST8000VN004-3CP101_WWZ6KWYX size: 7814026532 
    May  7 13:23:20 RatCreek kernel: md: import_slot: 0 wrong
    May  7 13:23:20 RatCreek kernel: mdcmd (2): import 1
    May  7 13:23:20 RatCreek kernel: md: import_slot: 1 empty
    May  7 13:23:20 RatCreek kernel: mdcmd (3): import 2 sdb 64 3907018532 0 ST4000VN006-3CW104_ZW624CEQ
    May  7 13:23:20 RatCreek kernel: md: import disk2: (sdb) ST4000VN006-3CW104_ZW624CEQ size: 3907018532 

So my intuition just keeps digging me deeper without improvement, time to try my first forum question.  I don't find articles that seem to meet my needs so how do I get back to a working array with the new 8TB disk as parity and the old 4TB parity to be used as data?  As an aside, I'd heard more than once that newer UNRAID versions don't care where you move a disk, but I'm starting to suspect that is not necessarily true in all cases.  Also, IDK what UNRAID means by a "slot".  Suggestions?  I have Diagnostics from 4/17 before I screwed things up, and several from the interim reboots described above.  I've attached the lated Diagnostics and a screen shot of Array Devices.

ratcreek-diagnostics-20250507-1433.zip

You cannot replace parity with disk1 missing, what happened with disk1? Also, is old parity still available?

1. disk1 was my samsung SSD which I removed and has been my cache drive for the past year.  
2. Yes, the old parity is still available, it is ST4000VN006-3CW104_ZW6280MG.  Though it is currently all plugged in, it does not show in Array Devices.  I want to repurpose it as a data disk.

10 hours ago, shakyknees said:

disk1 was my samsung SSD which I removed and has been my cache drive for the past year. 

This means that you have been running the array unprotected since then.

 

Check to see if old parity is detected in the BIOS, if it comes up you can do a parity swap.

10 hours ago, JorgeB said:

This means that you have been running the array unprotected since then.

 

Check to see if old parity is detected in the BIOS, if it comes up you can do a parity swap.

The Parity Check has been reporting success weekly for the year since I shifted that disk.  IDK whether it matters, but I had never written data to the array at the time I moved that disk to cache, during my first week of configuring Unraid.
 

• 

I just now shut it down and re-seated the connectors for the disk (ST4000VN006-3CW104_ZW6280MG) that was the old Parity until it rebuilt parity on the new 8TB, that you were asking about whether BIOS detected it.  It is now shown as Unassigned.  This is how it looked at one point in the earlier timeline, before I attempted to add it as disk1 (trying to use it for Data and make Unraid stop complaining about not having a disk1).  
 

• 

 

• 

Attached Diagnostics (ratcreek-diagnostics-20250509-1023.zip) from after the latest reboot.

ratcreek-diagnostics-20250509-1023.zip

10 minutes ago, shakyknees said:

The Parity Check has been reporting success weekly for the year since I shifted that disk.  IDK whether it matters, but I had never written data to the array at the time I moved that disk to cache, during my first week of configuring Unraid.

That's good but the array was still unprotected, if another disk2 failed you would lose all the data.

 

10 minutes ago, shakyknees said:

It is now shown as Unassigned.

You can now try a parity swap.

 

Reviewing my timeline, what I had done earlier this month was following the procedure in that link.  I had gotten thru Step 4 (in the section that starts with,

"The steps to carry out this procedure are:

Note: these steps are the general steps needed. The steps you take may differ depending on your situation. If the drive to be replaced has failed..." 
...through Step 14, which said it successfully copied Parity, but did not start the Data-Rebuild which Step 14 says should automatically follow:
"When the copy completes, the array will still be stopped ("Stopped. Upgrading disk/swapping parity.").

The Start button will now be present, and the description will now indicate that it is ready to start a Data-Rebuild._"
...but rather the Start button was not available due to the year-old latent "Disk 1" slot vacancy equating now to "Too many wrong and/or missing disks!".  Which is where I am again now.  
Note that Step 12 had told me to do what I did: "Assign the old parity drive in the slot of the old data drive being replaced You should now have blue drive status indicators for both the parity drive and the drive being replaced"  ...before I booted to start the successful Parity build on the new 8TB disk.   

After re-seating the previously-parity disk connections and booting, it showed up as "Wrong" under Parity disk, and I was able to select it to replace "Unassigned" for disk1, as intuition had led me to do a few days ago, but this time I rebooted the server at that point.   Now it said, "Upgrading disk/swapping parity.        Copy will copy the parity information to the new parity disk." and when that finished, I rebooted again and it started copying to Data disk1.  So everything is Ok, and I have no idea why it didn't behave this way the first time through.  Maybe I failed to reboot as required between steps. 

So anyway, all good, thanks much JorgeB, I owe ya at least a coffee.