Will upgrading my Mac result in the loss of my data? I am currently running on macOS Sierra 10.12.6, and I want to upgrade it to the new macOS Catalina. 


If I upgrade the OS without backing it up will I lose data?? 
And is it necessary to back up? 
What will happen if I upgrade to macOS Catalina without backing up?
Normally, nothing untoward will happen.  

However, nothing is 100% certain, ever.  

Backups are a necessary part of life. You really, really should always have at least one, preferably two; one being off-site.