Yeah, I still can back it up and copy-paste it again later. But that's not the point. It's the attachment, the inability to let go. That's the root of suffering, like what Buddha said.
Now that I already have all my data backed up in Time Machine, I still haven't copy-paste all files. I only keep what is necessary.
Scanning through all folders, I just realize something. Actually, the evil roots in the download folder. Such a mess and full of junk! Sooo.. lesson number one: keep it always empty. Delete files as soon as you no longer need it, Del.
Thank me later.