Marco S writes about his Time Machine backup filling up and failing to erase old backups and thus failing to be able to backup any more.
Time Machine has a history of bugs (very bad bugs like dropping volumes out of the backup list), so it is extremely unwise to rely on Time Machine as the only backup.
Tip #1 Always keep AT LEAST two clone backups away from the computer
Relying on Time Machine as a sole backup is risky indeed. For starters, if the machine is stolen or is destroyed (fire, flood, lightning strike surge), an attached Time Machine backup is also gone. So at least 2 backups that are stored away from the computer are mandatory.
I personally go much further: I have several always attached backups for daily usage (no excuse to not backup), as well as numerous backups stored away from the computer. The always-attached backups are best done with the OWC Thunderbay 4: 4 bays some of which can be main storage and some backup.
How many and where is a decision that relates directly to the value of the data being backed-up. For example, I carry critical business data like source code on my person when traveling, even in the mountains (in my backpack), because it would take years to rewrite that code—I’d be out of business. An SD card or thumb drive can serve this purpose.
I also carry my ongoing new work in my backpack when hiking in the mountains—I don’t want to return at the end of the day and find a broken window in my photography adventure van with my computer gear gone and with it 1/2/3 weeks of images and other work.
Tip #2 Erase the Time Machine backup volume every 3 to 6 months
Time Machine accumulates deleted files and versions of files. The last thing I need or want is 18 versions of a 4GB Photoshop file, or 73 versions of a Lightroom catalog file, etc.
Really big files can chew up enormous amounts of space over time, if edited regularly: Time Machine makes hourly, then daily, then weekly versions, which can lead to dozens of versions.
The easiest way to clean up the detritus is to erase the Time Machine backup volume every 3 to 6 months (using Disk Utility). Then let Time Machine start over (make sure it continues to use that volume, generally it should prompt/ask, but double check to be sure). Don’t do this erasure unless you also have those clone backups discussed above!
Tip #3 Don’t waste drive space—partition overly large drives
If following tip #2 and the drive is large enough, partition the drive into one volume for Time Machine use, and one volume for a clone backup. For example if there is only about 1TB of data to be backed up, partition a 4TB drive into two 2TB volumes.