The summary.
A SUBSTANTIAL PROPORTION OF ACCESS PROBLEMS ARE PERMISSION PROBLEMS
Check which account, which groups, what permissions apply at that location, and whether inheritance is involved.
Assign permissions to groups, never to individuals — per-person permissions become unmanageable and are never cleaned up.
WHERE THE BOOT PROCESS STOPS INDICATES WHAT IS WRONG
Nothing at all: power or hardware. Firmware then nothing: boot device. Kernel loads then fails: drivers or file system. Reaches login then fails: profile or services.
Safe mode distinguishes core faults from driver and software faults.
HEAVY DISK ACTIVITY WITH LOW PROCESSOR USE USUALLY MEANS MEMORY SHORTAGE
IDENTIFY A PROCESS BEFORE ENDING IT
Ending the wrong one loses work or destabilises the system. And disable nothing you cannot identify.
LEARN THE CONCEPTS, THEN THE SPECIFICS
Processes, permissions, file systems and networking transfer between systems. Command names and file locations do not, and change anyway.
THERE IS NO UNDO ON THE COMMAND LINE
Understand every part of a command before running it, especially with wildcards or elevated privilege.
