Boot issues can be some of the most stressful problems to deal with because you can't even get into the OS to run diagnostics. I've developed a toolkit of boot problems solutions over the years, but I'm always looking to expand it.
What are your most effective boot problems solutions for Windows, Linux, and MacOS? I'm interested in both software-based solutions and hardware diagnostic approaches. How do you determine whether a boot issue is likely to be hardware-related versus software-related before you start trying different fixes?
For boot problems solutions, I always start by determining if I can get to any recovery environment. On Windows, that's holding Shift while clicking Restart. On Linux, it's usually the GRUB menu. On Mac, it's Command+R.
If you can get to a recovery environment, you have access to tools for system file repair, disk checking, and sometimes even registry error troubleshooting (on Windows via registry hives in recovery).
My rule of thumb: if the system POSTs but won't boot, it's usually software. If it won't POST or shows hardware error codes, it's hardware. But there are always exceptions.
Windows boot problems solutions often involve the BCD (Boot Configuration Data). The bootrec commands in recovery are essential: bootrec /fixmbr, bootrec /fixboot, bootrec /scanos, and bootrec /rebuildbcd.
For Linux, knowing how to use grub-install and update-grub (or distribution equivalents) can fix many boot issues. Also, checking /boot partition space - I've seen many boot failures simply because the boot partition was full.
For Mac, Internet Recovery (Command+Option+R) is incredibly useful for boot problems solutions when local recovery isn't available.
On Mac, one of the most useful boot problems solutions is resetting NVRAM/PRAM (Command+Option+P+R at startup) and SMC (System Management Controller). These resets can fix all sorts of weird boot issues that don't have obvious causes.
For hardware versus software determination: if you hear the startup chime on Mac or see BIOS/UEFI screens on PC, but then nothing happens, it's usually software. No chime/no BIOS usually means hardware. But as mentioned, there are exceptions - I've seen bad RAM cause boot loops that look like software issues.