Drobo sees drive as bad on one mac andnot another

I have just shut down my drobo running fine on an imac and then connected it to my macbook pro. When it connected it said that 1 drive was faulty. I then plugged in a usb2 drive to the macbook and the drive went green and all was well. I unmounted the usb drive and the bad drive went bad again. I then connected it back to the imac and all was well again.
What on earth is going on?

btw it it connected via firewire

are they all plugged into the same power circuit? could be a ground loop type issue i guess.

the drives/drobos were all directly connected and not via a hub or similar?

All separate on everything

I’m still getting the same problem. Everytime I connect the drobo to the laptop it says a drive is bad until I plug in a separate external drive and then the drive becomes ok. Unmount the separate drive and the drive goes bad again.
Plug the drobo back to the imac and all is ok, well I hope it is anyway