When starting the Picocalc, the screen bottom message shows ‘Stabilizing SD Card’ - that does not sound very encouraging - what does this mean and what does the Picocalc do?
Then the boot page has the various environments. Upon choosing picomite for example, the screen bottom message shows “STAT: flashing firmware”. What is actually flashed, from where to where? As flash memory has only a certain number of writes before becoming unreliable, is that a concern? Better store a backup of the original SD card?
It’s doing literally nothing - the stock menu is just waiting for a couple of seconds before talking to the card. My replacement uf2loader doesn’t bother, it just retries until the sd card says it’s ready.
What is actually flashed, from where to where?
It’s taking a firmware image from the sd card and writing it to the flash of the pico, which is where it’s executed from.
As flash memory has only a certain number of writes before becoming unreliable, is that a concern?
You would have to reflash the pico hundreds of times a day for years to hit any problems, so don’t stress about it. And given that pico’s are dead cheap and the picocalc is explicitly designed to let you swap it out, even if it does go faulty it’s not a big deal.