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My iMac Big Sur 11.6 time machine backup failed repeatedly. I let it run at night (Oct 16, 2021) and in the morning I checked: my last successful back up was in February 2021. Rebooted and tried again several times. No error message appears.
iMac Storage report is 1.3 GB available out of 1.01 TB. Trying to backup to a Seagate external disk drive with 3.4 TB available.
iMac 21.5″ 4K, macOS 11.6
It appears that your iMac storage is critically low at 1.3GB out or 1TB. If macOS does not have enough storage space to create backup snapshots, then Time Machine backups fail.
It's imperative that you always leave enough free space available on the startup drive. By rule of thumb, that will be 10-15% of the total capacity of the drive.
You will need to offload a good chunk of your user data to another drive. The files that generally take up the most space are audio, video and photos. Your Photo library can be relocated to an external drive, as can your Music library.
Move your Photos library to save space on your Mac - Apple Support
Change where your Music files are stored on Mac - Apple Support
It appears that your iMac storage is critically low at 1.3GB out or 1TB. If macOS does not have enough storage space to create backup snapshots, then Time Machine backups fail.
It's imperative that you always leave enough free space available on the startup drive. By rule of thumb, that will be 10-15% of the total capacity of the drive.
You will need to offload a good chunk of your user data to another drive. The files that generally take up the most space are audio, video and photos. Your Photo library can be relocated to an external drive, as can your Music library.
Move your Photos library to save space on your Mac - Apple Support
Change where your Music files are stored on Mac - Apple Support
Welcome to the Apple Support Communities.
iMac Storage report is 1.3 GB available out of 1.01 TB.
Regardless if this is related to the issue or not, you need more free storage. 10-15% at least should be free at all times.
Please see: Free up storage space on your Mac - Apple Support and Optimize storage space on your Mac - Apple Support.
You may also need to store large files and libraries on a large external storage device. Please see: Use external storage devices with Mac - Apple Support.
Depending on how much storage you might need, one of these external storage solutions might work for you:
After you have a good amount of free storage available (at least 10%), try to backup again.
Jack
back up mac 11.6 failed
