First thing I'd do is check all the hard drives for errors... weak sectors can take a long time (I've seen one go as long as 5 seconds per sector to read on a drive that eventually completely failed) to read and write.
If the check does turn up any bad sectors, step up the frequency at which you do drive checks... if more of them pop up as time goes on, start thinking about cloning that drive, removing it, dropping in a new drive, then writing that .IMG file clone onto the new drive (all of which can be done via the Disks application while booted to the Zorin OS BootUSB stick).