I have 1.5 TB of 2.1 TB currently used. Of that 1.5, 1.0 is "System Data." It's my understanding that I can delete all that. Is there any harm in doing so? Is there a risk I could lose data (e.g. documents, pics, etc.) that I don't want to?
"System Data" is just as it's labeled... data that the system - macOS - requires, uses and controls.
Yes, there is great harm in deleting system data, up to and including bricking your Mac and making it unusable. This is one reason that the newest versions of macOS are installed on locked, read-only APFS drive volumes.
Your personal files - documents, pics, etc. - live in your user Home folder. This is accessible to you and held separate from the macOS files.
For additional info, please see: What is “Other” storage on a Mac, and how… - Apple Community
"System Data" is just as it's labeled... data that the system - macOS - requires, uses and controls.
Yes, there is great harm in deleting system data, up to and including bricking your Mac and making it unusable. This is one reason that the newest versions of macOS are installed on locked, read-only APFS drive volumes.
Your personal files - documents, pics, etc. - live in your user Home folder. This is accessible to you and held separate from the macOS files.
For additional info, please see: What is “Other” storage on a Mac, and how… - Apple Community
Please just leave that alone. If you are concerned with storage space then the wise thing to do is to move large libraries to an external drive. What tends to take up the most storage for most users are the following types of library files:
Moving them is simple and Apple explains how to do this in Move your Photos library to save space on your Mac - Apple Support . Obviously this is the process for your Photos libraries, the process is the same for music and graphics.
Once you have moved the libraries to an external drive simply click on them and that will "teach" the app that uses them (for example the Photos app for the Photos library) where the library has moved to. It's that simple. Once you have completed that step you can check where the old library was and then delete that and empty the Trash to free up your space.
Finally if you need help finding a good high quality external drive many experienced users prefer www.macsales.com products. You have a LOT of options available to you, if you don't know what you need then call them, their service is great.
Deleting System Data from storage.
