What are the risks of losing what's on my hard disc--especially programs--when I update Yosemite?
Low as this does get tested before.
What is not tested as much is does each application work with the new OS. Apple can only test major apps it is the responsibility of the App's developer to test their app with the new OS and not all have the time to do this.
But as always with any computer change take backups as there is always a chance of something going wrong.
I assume that changing something will break something else. When it does I have to work out if it is worth living with that problem or revert to the old version.
Another example is professionally in enterprise software when I install a new version of my program I have to provide a tested process that will reverse the change as stopping something working can cost money
The risks to a system being able to boot again are highest when you make fundamental changes to the OS as part of an upgrade.
This is due to three factors:
- some parts of the code base are only called when swapping in and out a new system so those code paths don't run normally. Any degradation of the files (bit rot) will be discovered, but not necessarily caused at upgrade time.
- any change has a very small chance of failing - more change, more chance
- you run new migration steps from the vendor and they could run poorly or not anticipate the changes you have made to your system and cause data loss. This is mitigated by storage idioms like putting all user settings and app data in folders where the OS doesn't install any files. (i.e. the system is designed to not break thusly)
The risk to your data files is no higher during an upgrade than otherwise. If you don't care to back up the files anyhow, then you're always one disk error or drive failure away from losing data. If you have a good backup, you mitigate all the risk of upgrading other than the time you spend wiping and reinstalling the old system / old backup.