Time Machine no longer sees WDMyCloud backup disk, even it's seen in the finder

[jerone]

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I couldn't remember password to mycloud, could only log in as a guest, it wasn't backing up so I did a 40 second reset to change the password. Now says backup disk not available but I see it.

 

[Rod]

In most cases, including this one I think, a WD External HD will be formatted for Windows not macOS. That does not mean the Mac cannot use it but unless you need to share the drive between a Mac and a Windows PC it should be erased/reformatted as GUID map for MacOS Extended (Journalled). This will remove any programming that came pre installed including any built in security meaning no need for passwords.
Assuming you can see the device in the native Disk Utility app you can easily erase it as above and name it at the same time.
Go to Time Machine and select the new drive name as your Time Machine backup and let it run. It will take a while.
I have several WD My Passport's and I have had to do this to all of them.

 

[MacInWin]

No, the MyCloud WD drives are networked, so the format doesn't matter. TM should do a sparse bundle backup to them. The drive is not directly attached to the Mac, but accessible over WiFi or Ethernet, though the central LAN router. I've never had to format a WD My Cloud drive. Rod, I think you are thinking (and said) WD My Passport, which is a direct-attach drive that DOES need reformatting to HFS+.

Jerone, try shutting down the Mac, then power off and back on the WD MyCloud to get it to reattach to the router, then restart the Mac and see if it can connect to the WD MyCloud drive as a networked drive.

 

[Rod]

Oh, I didn't pickup on that. Sorry.