Guerin wrote:
Time Machine Backup Failed, because the hard drive, SSD, I want to backup is too full,
yes, TM needs to make a "snapshot" of your data on your local drive BEFORE it gets sent to your TM drive. if you don't have enough space, TM cannot do what it needs to do to create your backup.
if you are running low on space, you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.
additionally, you want to keep ~ 20% of your local drive as free space for proper operation of macOS.