Guys, you're right. Here's more details:

1. Fan is there, it is working, and it is as noisy as always. I've been planning to replace it for a while, incidentally, with a quieter Noctua.

2. WD 1TB, 2TB and 3TB Green drives. Various subversions of them

3. I am afraid Drobo dashboard does not report drive temp.

4. Not sure HOW they fail. RED led next to the failed drive. I've just been replacing the drives without further investigation.

5. Failure rate: in 3 years, I replaced possibly 4/5 drives.

6. Admittedly, these are all drives I've previously used on my server at home, for at least 1 year. As I upgrade the server storage capacity, replaced drives end up in the Drobo, so none of its drive is new.
One could argue they would've failed in the server at some point.
However, other than a recent 8TB drive failed on me (see another thread on this board about drive temperatures), I have not had a drive fail in my server for 10 years at least, or more.

Matt, yes, I've been using Synology NAs units at work for various projects and, at least those I've been working with, they seem much greater quality than the Drobo. Mostly just an impression I get, admittedly, maybe because I like Synology OS so much better than the extremely basic Drobo's, but still.