In the mid-2000's I briefly worked for a company that did this at a firmware level ("write-blocked firmware") for USB drive adapters (IDE / SATA / whatever IDE variant laptops were using / etc). This was apparently very valuable for police and investigative services, so they could collect evidence, while being able to show that they did not tamper with the original drive.
Tenable makes some "read only" adapters for hard disks (SATA, PATA, SCSI & FW at least). They're usually sold as part of a forensic analysis kit. I have a couple and they definitely work. I believe there are a couple of other vendors (Wiebetech?) make similar devices.
The alternative (tho not practical in many cases) would be RO media like RW-DVD.
You mean DVD-R? DVD-RW is rewritable, which means it's not really RO. The semi-obscure DVD-RAM takes this a step further by making it work a lot like a hard/flash drive (at the user level, not technical).