What Drive to get for as replacement for Hitachi 5800rpm in 5C

Hi. In my 5C dashboard, I’m getting an orange ‘Warning’ for Bay 4, although on the Drobo unit itself all drives show solid green.

Here’s a screenshot from the dashboard:

I wonder if I should replace that drive?

All the drives are Hitachi, spinning at 5800rpm. I’ve read that, ideally, they should all spin at the same speed, so I should try to find a 5800rpm one. Not sure if that’s possible.

All suggestions are welcome!