I recently bought a couple of old single-core B+ units on ebay. One of them is weird - it always boots into the recovery partition (directly from the rainbow screen, not via the 'press shift' screen). It is the Pi itself that is causing this behaviour,, not the contents of the SD card - same thing happens with other SDs (raw noobs or installed noobs), and it doesn't happen when I swap out this B+ pi for the other one, keeping everything else the same (i.e. eliminating issues from PSU (3A) or keyboard/mouse (USB) or display (HDMI)).
It's actually quite impressive to see a Pi booting to the unix command line within a second of powering it on! However I would like to understand what is going on and if it can be fixed? (Also if it can be recreated, should I actually want a really fast boot environment of my own some day )
Is it likely that someone updated the boot firmware with a broken version at some point?
If it is a broken firmware issue, I'm guessing I have to boot up full unix to be able to run rpi-update to rewrite the firmware?
How do I boot into the full unix environment from the recovery partition? Pressing ^D doesn't do it. I presume I have to mount/remount all the partitions manually, but having done that how do I reboot into the proper environment without losing the mounted partitions? (It's been 20 years since I had a linux system so broken I had to do anything like this and that was back in the BSDI days... things have changed a lot since then...)