Problem solved! Swapped out the new WD 2Tb HDD and put in a new Samsung 850 Evo SSD. Once buttoned up, the drive was seen and MacOS initialized it without a problem. Data is now being moved from the new WD 2Tb HDD that I put into a USB cradle. It as an external drive but the EMC2390 doesn't like it internally because it's SATA III with 6Gb throughput.

Be advised. The guide above says that as long as you stick with the same manufacture, it will work. 7 years later, that is no longer always true. Make sure the SATA versions match up too.

I also used disk utility to see if I might be able to reformat the new drive now that it's internal. DU didn't see it either.

I had previously added a 256Gb SSD to the OEM WD1001FALS. Heat issues started recently so swapped in a WD2003FZEX 2tb for the OEM 1tb WD. Because cracking the system is the hardest part, I also swapped out power supplies with one from iFixIt.

On reboot everything else is fine but it doesn't see the WD2003FZEX drive. Tried the cmd-r, the cmd-opt-p-r, and other resetting options with no luck. Cracked open the system again and confirmed the HHD cables are all in right. Still nothing.

Internal SSD boot drive: Fine.

External connectivity and everything else: Fine.

I had pre-formatted the HHD using an external USB cradle and copied my data over in advance of the swap (the OEM HHD was being used for data and large apps not for boot). So I know it was working.

re: WD Green drives are meant as desktop drives and do into error correction mode at times when a NAS is expecting a result. When the Green (or Black) time out for these NAS requests (because they're busy) the Drobo will flag it as failed. I had it happen too. To resolve that, WD has released their RED line of drives that are meant for use in NAS systems and don't "go away for their own housekeeping" like the Green/Black/Blues do.

I have a 2010 iMac 27" and did a CPU upgrade from an i5 to an i7-860 as well as installed an SSD using iFixIt's Dual drive kit and guide. They rate the dual drive install as difficult and 45 minutes. The CPU mod requires more R&R yet is rated moderate and up to 1 hour. First time into an iMac (but with decades in hardware/software support) and the total project took 3.5 hours. I'd keep this at 1 hour and bump the SSD install to 1 hour with a caveat that it could take longer for first timers. I'd also bump this up to "difficult" to match the easier SSD install guide rating.

My temps are running fine on the system post-mod but the fans do run quickly (but not loudly since the loudest thing in the room is the NAS up on the shelf across the room).