Time Machine external drive backup fails: insufficient space

Most any drive will work with Time Machine, until it doesn't.

Your TM backup drive has reached the limit of its capacity and TM is doing what it's supposed to do; remove older backups to make room for newer.

TM backs up any files that change over time, therefore the need for more drive space than the source being backed up. Your source may have 3.36 TB free, but the TM destination surely does/did not.

Since TM has erased old backups from the destination drive you can allow TM to continue using that drive as is. If in your situation, I would manually reformat the drive and start a fresh backup set with TM.

To be honest, I would follow the guidance provided by Apple and use a drive that is at least 2X the capacity of the source.

Back up your Mac with Time Machine - Apple Support

❝Use a storage device with at least twice the storage capacity of your Mac. For example, if your Mac has 1TB of storage, your backup disk should ideally have at least 2TB of storage. If Time Machine needs more storage space, your Mac will let you know.❞

Time Machine is designed for the startup volume, or volumes where there is significant data churn. That way "Time Machine" keeps a backup of your data before you realize that you need it.

But Time Machine is not suitable for a 12 TB drive. There's no way you can churn that much data. Just copy the drive using other means. It'll be faster too.

I have seen in people say in various forums that the TM requires a drive with at least double the space of the drive being backed up, but this is not my experience. I have always used identical drives, and until last night it has always worked.

So in other words, people were eventually proven correct and you want to know why?

It's like when people tell you not to jump off the roof. You it anyway and don't break your ankles. Then one day, you jump, and your ankle shatters. Don't blame the roof.

JamieFarqy wrote:

Last nights back up failed, I got a message that there was insufficient space on the back up drive. It appears that all the previous backups have been erased, and TM is starting from scratch.

Confirming that the message referenced the external drive. Typically, TM will erase old backups to make space for new ones as needed. The only time I've seen an insufficient space error for a TM backup was when the internal drive was low on space.