They have firmware, and it can be updated via special programs. That firmware is not run by the CPU and it's not present in CPU addressable space (so you won't be able to dump it using debug).

In most cases, the firmware can't be dumped with the computer... it can be only read with flash or eprom programmers.

Disk firmware releases are pretty rare in the consumer space.
Optical drives did get firmware updates, some of them quite some. Usually burners, to support new discs.

In the past there were many sites with firmware for CD-burners (sometimes modded). One of those that remained is www.cdrinfo.pl - polish site with 2813 firmwares. Here is search engine: https://www.cdrinfo.pl/biosy

Hard disk firmware is generally only released when a specific bugfix is needed - Seagate have a pretty good serial number check, and some Seagate drives (Barracuda 7200.11) have a particularly serious bug.

Firnmware for that slot loading Plextor is also very thin on the ground, and Plextor only seem to mention SSD these days

As mentioned before, both have firmware but use special software to access these. While hard drive firmware usually is only distributed by big OEMs like IBM, HP or Dell, optical drive firmwares are often availiable at the manufacturers site. I had same nice experience with Plextor: They sent me an updated BIOS chip for my drive, but I had to remove the PCB and desolder some drive power lines to reach the socket to install it.
eisapc

Thank you all for the replies!