I use a 2.5" drive. I find them ideal. But then again my backup footprint isn't double-digit terabytes.

Many of the large 2.5" drives are SMR, and I've found sustained write storage go be mediocre at best - 60-80MB/s.

Yes, and it doesn't in the slightest impair their ability to keep my data.

It does impair your ability to transfer it inna reasonable timeframe though! Filling a 4TB drive is a 20 hour endeavour at those rates!

What's reasonable will of course differ from one person to the next. Even if I had a full TB of data to back up regularly (I'm glad I don't) it's for me perfectly alright to let this simmer in the background while I do something else, because I don't need to actively oversee the transfer and I don't need it to be done in a hurry.

In another post of yours in this thread you mention that you have gone "all in on M-Disc" for your archiving, which come with much lower transfer rates (they typically max out at ~16 MB/sec last time I checked) and the requirement of actively managing disc swapping along the way. I'm sure you can appreciate the far more efficient option of a slow 2.5" SMR drive.

I appreciate this might seem contrary, but I see it like this: if I am using hard drives, I am using them for speed. If hard drive is only double the speed of Bluray, then its not a sufficient differentiator for me.

Enterprise grade, CMR 7200+RPM hard droves, on the other hand, are closer to ten times faster than uray, so more worth it.

The post title is wrong and was poorly transcribed from the article. The product is named M-DISC.

It is a convention that optical discs are discs, and floppy disks or hard disks are disks. It is not a hard-and-fast spelling rule, but since there is indeed a set of official standards for Compact Disc and Digital Versatile* Disc, you can be sure that any conforming optical disc won't be spelled with a "k".

* Were you surprised that the "V" doesn't stand for "Video"?