kobo sage with koreader and aftermarket magnetic flip cover
Discussion
I may have to look into this. My failing unit is a kobo, not a sage, and the failure mode is one I suspect could easily be fixed by switching to a new firmware.
koreader doesn't really replace the firmware of the entire device... it's more like an "app." it shows up as a normal book in the kobo's interface and when you open it, it tricks the machine into executing a bunch of code and launches into a newfangled environment. the stuff in there is better, like a real hierarchical file browser that explores the contents of the storage. the reader is also better. if you shut the device off completely and turn it back on you have to go into the "app" again to make it come back.
Even that may work. The problem I have is when I start a highlight the screen locks out all input. It takes minutes or a full reboot to restore. That would be avoidable except I don't have page turn buttons so it often highlights when I tried to turn the page.