Welcome.
I'm not readily aware of a method to flash, nor repair HDD, for the JVC DR-MH30.
But you have to remember I was using these 20 years ago (when new, 2005), and slowly backed away from then across the 2010s. In the 2020s, my units are off 99% of the time. Don't get me wrong -- they're excellent! But the need to make DVDs has collapsed. In fact, if I used it for DVDs, I'd just turned around and rip, then edit! The only saving grace is XP mode, for tapes that need "quick convert to DVD", as it does have chroma NR in the LSI. So it will give the best results in this now-uncommon scenario. Although, I must add, that a DVD recorder is far preferable to the junk Easycap/Elgato/HDMI-dongle methods used by too many.