I have an iMac running High Sierra 10.13.6. My internal Drive is about full and I've been looking for ways to clear up some space. My internal drive has a 250 GB capacity and I have around 15 GB of free space. The largest folder on my System is the Library folder listed at 134 GB which appears under my user account. This seems extremely large to me. Inside this Library folder is the Mobile Documents folder at over 91 GB. The largest item in my Mobile Documents directory is a folder named MyBook, which is the name of my external drive. Everything else seems of reasonable size. My question is 2-fold. Why is my 'Library Folder' so Large and is there anything I can do to decrease the size of the Mobile Documents folder.
~/Library/Mobile Documents is the aliased location for iCloud Drive. You can access this from the sidebar or
Go > iCloud Drive ⌘⇧-I. If you have a folder in there with the name of your external drive, my guess is that it has been copied by mistake or perhaps a 3rd party backup? I can't think of any built in system that would do that.
For other files, you can see exactly what is taking up space for Mobile Documents in the iCloud System preference. Once in that preference pane click on Manage... bottom right. This will show you all of the programs that are storing data and you can delete them as you see fix. Keep in mind that some programs require a certain amount of data that you can't delete.
If you don't want a program to store data this way, go back to the main iCloud pane and click the Options... button to the right of iCloud Drive There you can check/uncheck the programs that are allowed to save data to iCloud. Also take note of the Optimise Mac Storage checkbox, you'll want this turned off.
Mobile Documents Also stores photos that have been shared to you from other people with iCloud Photo Sharing, you might want to check this option in the iCloud preferences too.