Preparation for MBP Retina mid 2015 SSD Replacement Upgrade

TimeMachine will create a complete backup of your apps and data to your external drive.

Making the effort to also create a iCloud copy of your important stuff is not a bad idea. I find having an iCloud drive helpful when I need to access information from multiple devices while on the road. But keep in mind your network connection will hold you back! Most ISP offers for residents are asymmetric connections. Think of it this way you have a dirt road to get to your office (25MPH) in town and a highway going home (75MPH) so it clearly take you less time getting home! Now think about all of your travels how much time you waste getting to work! If you can afford it a business connection will likely be a symmetric connection offering equal speed in both direction, but that is costly!

While having a bootable external drive (USB thumb drive) is very helpful its not really needed. Your system has a powerful feature offering internet recovery and upgrades. Even still I would make one just in case!

Using Disk Utility format a 32GB thumb drive with GUID and a journaled file system, Download the OS installer file from the Apps Store (or from How to get old versions of macOS Then follow this guide How to create a bootable installer for macOS. Double check you can boot up with it.

So the last part of your question what’s not backed up? If you’ve apps from Apples App store you may need to reauthorize them as the drive ID which was used is now different. Some other Apps may have other means to prevent pirating.

And lastly the malware and viruses you still had on your system! But you could pollute your new system if your not careful! Make sure you’ve install and run a good antivirus and malware app before you even start!