I seem to have fixed the problem on my RPi 2. I was using OMV for Raspberry Pi release 2.0.15 and I saw there was a newer release, 2.2.1 available. I replaced the old image on my MicroSD card with 2.2.1 and after that change, the Error #4000 stopped when I mounted my HDD. The setup of my system went perfectly after that.
It is also possible that simply reinstalling the original OVM release 2.0.15 could have fixed the problem. I did not try that, so I don't know if it would have solved the problem or not.
Anyway, I thought I'd post this here, in case others see the Error #4000 on HDD mount in the future.