Due to an HDD failure, I have purchased and installed a new Seagate BarraCuda 1TB 3.5-Inch SATA III 6 Gb/s Internal Hard Drive (ST1000DM010)(https://www.amazon.ca/gp/product/B01LNJBA2I/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1).
The Mac is a 27" iMac EMC 2390.
When I boot using a System Install USB stick and launch Disk Utility, only the USB stick's device and partitions are visible. The unformatted HDD is nowhere to be seen.
The original drive was a 1TB Seagate; I see a slight discrepancy on the power specs, but not sure whether this has an impact.
Anything else I should try? At the moment I don't have any means to independently verify the integrity of the new drive (external Caddy etc).
Based on feedback here, I purchased an HDD caddy, which connected my new HDD to a working laptop via USB. I was able to mount and format the drive, and even proactively installed High Sierra on it. This at least confirmed the issue was not with the drive.
So then I proceeded to do a motherboard removal to get at the SATA cables behind it. I did not find any issues with the cables, but removed them, blew in them to clean them out, and reseated them before reassembly.
Upon reassembly, the computer recognized the drive and booted at once into High Sierra - success!
The process was relatively painless, though not complete free of issues: I snapped a fan housing (when I dropped the fan), which had to be epoxied, and I busted the JST connector for the
skin sensor 2-wire cable, which I managed to hardwire upon reassembly. Now, although the Mac works, the fans are on full-speed all of the time, making it sound like an industrial shop vac and pretty much useless in the office environment. Live and learn.