Assuming common-law norms:

Are the police liable for anything here?

No. The report is accurate to the best of the officer's knowledge at the time. There are policy issues with an officer telling someone not to answer a question like "why did you punch me" on the spot, but not legal issues, as such.

It likely isn't admissible in a civil suit to recover medical costs, because it was written by someone who did not witness the events and is therefore of questionable relevance, but even if it is introduced and allowed as evidence, the plaintiff can make the point that the report is inaccurate and use whatever other evidence they have - including any witnesses they can find - to support their version of events.

Are the police liable for anything beyond the cost of the hard drive?

Generally not. Even if the plaintiff can show that the loss of the drive was intentional, it's not the police's job to safeguard peoples' data for them.

Having only one copy of information vital to a lawsuit is foolish in the extreme: any data you only have one copy of, you will eventually have none copies of.