I got out my torx wrench (need T-5 or T-6) and opened up my M-500 and M-505. I wanted to see what makes the on/off switch difficult to press and see if I could figure out why they were not charging well.
The M series started to have plug in batteries. I like the M-5xx series because they also use an SD and MM card. The processor is slower than the OS 5 series palms but OS 4 has no DRM protection for programs.
When opening the back panel be very careful not to break the plastic near the Stylus port. There are three plastic ridges on each side and pull up the back cover from the hotsync port.
The m505 uses a real aluminum back cover with plastic insert. The M500 back plate is all plastic. They both look similar but the M500 display is black and white only, while M505 has a color display.
I think I have burned out both of my M series devices. I accidentally used a 12 volt charger! One still remains on and displays gibberish lines. The other one turns off when off the charger (it might be the power button since it still has battery voltage.)
Th-55 U, T/X, TG-50, Handspring Deluxe (3) and PSP user.