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1System Storage Cleanup
2
3Empty Trash, Delete Duplicate Files, Check Documents for unnecessary stuff
4
5Delete local time machine snapshots: 
6
7sudo tmutil listlocalsnapshots /
8>gives
9com.apple.TimeMachine.[key]
10>insert
11tmutil deletelocalsnapshots [key]
12
13
14Go To Folder
15~/Library/Containers/
16Delete files. Nothing essential
17View > as list > View Options > Calculate all sizes
18
19
20Go To Folder
21~/Library/Caches
22Delete Files. Not all, only old & useless - some caches hold settings & preferences for programs
23View > as list > View Options > Calculate all sizes 
24
25Troublemakers: Mail Cache (500MB+), Spotify Cache (6GB+), Djay Pro 2 Cache (2GB+), Adobe AE Cache (6GB+), WhatsApp Cache (500MB+)
26
27Mail: 
28Erase all deleted Mail
29Erase all Junk mail
30
31Go on gmail.com 
32>Preferences
33>IMAP/POP Settings
34>Set Limit for how many mails are stored in IMAP folder to 1’000
35>Save Settings