in Mac OS 8 and 9, you could patch the system direct. OS X was harder, and became sprawling and interdependent, encouraging users to reinstall macOS. That has now ended with the introduction of the SSV.
When should you reset your SMC and NVRAM, repair permissions, install a Combo update, or re-install the whole of macOS?
Time Machine not backing up hourly? Troubled by repeated prompts to enter your user password? iCloud Drive playing up? Try installing the latest Combo updater. Updated for High Sierra.
Installing a Combo update of OS X is a popular panacea for puzzling problems. But you need to do a bit more to complete the job.
Sunday morning nightmare: how a missing component of OS X brought my iMac to its knees.