Installing OS X Sierra: Potential issues and how to avoid them.
There are posts that come across these forums that sometimes ask "why did Apple release a buggy OS" or "I'm reading about all these nightmares" from people having issues with OS X 10.12 (or some other mac OS X itteration)
The first thing everyone needs to take into consideration is simple: users not having issues will not post to say everything is working.
The second thing is this: With so many 3rd party software and hardware products not all of them are either tested by the developer, or supported any longer, or tested to find something specific in a specific workflow that could become a problem for someone else.
Apple does internally test their releases of OS X, and they test their bundled software, but more importantly they allow the software to developers and beta testers months in advance to test out. For the software developers this is a time to test what they offer in the new OS. The nature of any OS be it Windows, Mac or Unix is an update my change framework, or include securities that were not present in a previous OS. As a result developers need to identify those changes and work on a solution. This is the responsibility of a developer, not Apple. Apple can not destabilize their OS with modifications they can't test in time in order to make someone elses software work for them. While Apple sometimes works with developers like Adobe or Microsoft to assist in projects or issues with upgrades not all developers have this offered to them and the burden is on them, even if they sell in the App Store, they need to fix it.
Another inherent problem is some hardware devices may no longer be supported, again this is the responsibility of the manufacturer who has the OS from Apple months in advance to make that decision. If a 3rd party decides they no longer want to support their NAS or Printer or Scanner, its not an invitation for Apple offer software and take up the support of a retired device. All companies retire software, OS's and hardware, it is the nature of computers.
Apple states in the OS 10.12 End User License Agreement that any issue with 3rd party software is not something they will address. In the event you encounter such an issue you should contact the developer of this products (see section O)
http://images.apple.com/legal/sla/docs/macOS1012.pdf
Apple suggests you backup before any upgrade. In the event there is actually a problem either internally with from Apple or more often with a 3rd party title this gives you the opportunity to restore your computer to a time where the OS and your software worked together. Proceeding without this in place can be reckless and the cost of backing up a computer is a fraction of the time wasted and frustration gained from not doing it.
Use Time Machine to back up or restore your Mac - Apple Support
If you have backup in place you can either upgrade or clean install. In the event you are concerned with how the new OS will behave you can install the new OS on a partition of your hard drive or on a new drive entirely. A clean install will remove the potential for incompatible software from the previous OS and also keep a copy of your previous OS and software and files in the event you want to go back without having to restore from the backup (I suggest keeping the backup regardless) When installing the OS I recommend you disconnect all external devices not needed for the install. If you are on a laptop you really don't anything plugged in. With the exception of a Mouse and Keyboard for iMac, Mac Mini and Mac Pro users the only USB device you may want plugged in is the flash device you choose to put your OS X installer on if you did not install directly from the hard drive. Once you have installed the new OS you can restore your applications and data to the system if you choose to do so and test them in their new environment using OS X's built in migration assistant.
Move your content to a new Mac - Apple Support
or you can choose to install the 3rd party software form the developer and using their latest product updates. This is a preferred method, check with the developer to find what they claim for compatibility. While sites on the internet offer this information they are prone to misinformation from users who are not developers and who have not fully tested theses titles. While I'm not discouraging anyone from using these sites keep in mind their findings may not be the statements from the developers of the 3rd party title. Get a 2nd opinion.
The more you can test your new OS without these 3rd party software and hardware items installed one at a time, and the sooner you can see if one or more of these titles is the actual cause of the problem before you ask "why did Apple bother to release OS X 10.x if they knew..." and the problem was something else.
