Success! I've successfully updated player kernels to the latest Hijack 526.
On the Mark 2a previously mentioned, I was getting the hda: err: Stat_err message. Suspecting a HD fail I've tried to recover with Spinrite, but found no issues/errors. I've realigned the pins further and attempted again...this time it went, but in my exuberance, I selected the builder_bigdisk_v21.upgrade and wiped my content. :-(
I know there are explicit warnings that all data will be wiped, but any chance of some kind of a recovery with binwalk or some such tool? It was dumb, but I was so excited it was working I forgot I'd changed from new media back to my disk with content (I was initially trying to get a CF card and/or a M.2 SSD drive going in it and not being successful--which is why I had the builder_bigdisk in the first place.)
On the other (older Mark 2) player, I've installed the latest car2-v21.upgrade w/o issue. Shiny!!!
Huge thanks!!