You can replace the controller board all together, the electrical interface should not be that hard. You have to spin the spindle, jiggle the heads and send/receive the data. The main problem I see here is that modern hard drives have enormous, unimaginable information density. There must be so many tricks how they achieve that (and do that reliably!). You have to re-trace the steps the hard drive industry has made in the last 50 years.

> I would like to see a hard disk with all open source software.

You’ve got me curious of the use case, is it fascination or some other reason? From a security perspective, it seems straightforward to just encrypt the bytes before sending to the drive. That would ensure you can detect data being maliciously changed and/or protect against the drive somehow misusing the data.