I am not a lawyer, and more to the point, I am not a German lawyer, but to give strictly non-legal advise, at this point, I think it would be more than reasonable to contact a German lawyer to inquire about filing a civil suit against the police department for financial damages arising out of their repeated and unreasonable delays in returning your equipment or a copy of your data from that equipment.
Their delays are costing you actual money, their delays are far beyond what any reasonable person would consider acceptable, and it seems highly likely to me that a judge would agree with both of those statements and find in your favor. If they can't clone a hard drive in under a year, then the only reason they're holding your drives is for a fishing expedition, which is illegal under pretty much every country's legal code, including Germany's, as far as I know.
And to be blunt, the longer they take to return it, the more likely it is that you'll end up suing them anyway to get it back. But the doctrine of Laches (which AFAIK is also a thing in German law) will end up limiting the payout if you take too long to file such a lawsuit, because they'll be able to reasonably argue that by waiting to enforce your rights, you allowed the damages to accrue, so there's likely no advantage to waiting any longer.
There's no defense like a good offense. Just go on the offensive and be done with it. Enough is enough.