Intermittent and long cycle time failures can be difficult to diagnose, often impossible to be certain of the cause(s).

You may want to consider replacing this Drobo unit with a different unit or a different model. Retire or sell this one, as you deem appropriate.

The ongoing downside risks with the current Drobo in terms of your time and potential data loss probably outweighs the money value of the Drobo unit, not to mention the cost of replacing hard drives multiple times.

Sometimes it is best to just move on, leave the troublesome unit in the past.