I tried to get the SMART data, but could not via 3 different USB-SATA adapters, and one eSATA. As soon as I have a minute I am opening one of my PCs case and get it.

K447, thanks for hinting at all other possibile causes. Let's see
- Drobo electronics: no way for me to easily find out. I've been planning to replace the fan with some quieter one for a while, so I will eventually have to open the Drobo. I'll inspect it for anything suspicious.

- Drobo does vibrate itself at times. Apparently, many do, and by pushing the HDD firmly in (as recommended by Drobo themselves) it does help: vibration noise drops. Drobo is sitting on a Desk with my home PC, which is mostly off. it is connected to my home server, on a desk nearby, which is on 24H, but vibration from it do not reach where the Drobo is.

- a 650 VA APC UPS with a battery still in good shape is protecting both the drobo and the server.