Thank you very much, SgtSixpack! I like the $329 price (of that HDN721010ALE604 Deskstar). I think I'll do some digging, because I'm wondering how it's speed. compares to the Ultrastar HUH721010ALE604 $350
I just (oops, sorry!) remembered that for me Seagate's various 10TB models are out (I got an 8TB IronWolf in January, but Seatools for DOS didn't support it...and I'm firmly opposed to deploying HDD's for which I must run HDD diagnostics in Windows (and I was frustrated in my last attempt to create a WinPE with HD Tune on it).
WD Data Lifeguard Diagnostic for DOS v5.27 supports my 8TB Red HDD (when I first got 6TB Black drive v5.25 did not support it but I phoned and they posted 5.27 the very next day).
I wonder if HGST has it's own bootable HDD diagnostics, or if v5.27 supports the HGST drives. If not, maybe I'll be stuck buying an (ugh) WD Red (my current one is just an HTPC drive).
Edit: It appears that HGST's old Drive Fitness Test v4.16 does not support drives >2TiB, since I just tried booted to it and it thinks my 6TB Black drive's capacity is "1603 GB". I'll phone HGST Tech Support when they open on Monday and ask if they have a DOS HDD diagnostics (or if WD's) will support their 10TB HDDs.
If not...I'm afraid I'll be left deciding between WD's 7200rpm 10TB drives: Red Pro and Gold.