Hi everybody,
I have a 12TB Time Machine disk which is backing up an 8 TB disk (with 1 TB free). Notwithstanding this clear math (the BP disc is larger than the target disc) and the fact that TM should erase the older BUs to make room for the new one, I kept getting messages like "Back Up failed because the disc is full"...
How can I fix this?
iMac 21.5″, macOS 10.15
As large as 12 TB is, it is entirely possible it is insufficient for Time Machine.
The reason is that TM requires enough free space to create one complete restorable system backup, plus sufficient space to create at least one additional backup of information that has changed since the previous one, plus an additional amount of "overhead" that is not easily calculated (but a very approximate guess of 15% can be considered a minimum, and a lot more than that is possible).
In other words,
Notwithstanding this clear math (the BP disc is larger than the target disc) ...
That's not all there is to it.
.... and the fact that TM should erase the older BUs to make room for the new one,
TM only does that after successful completion of a subsequent backup. You can see it do that if you were to examine its logs. You have that backup. TM will not touch it until it creates a newer one.
The salient principle to keep in mind is that TM guarantees an absolute minimum of one and only one complete, restorable system backup. More than that is "nice to have" but not guaranteed.
Bottom line: if TM says it needs more space, then it needs more space.
As large as 12 TB is, it is entirely possible it is insufficient for Time Machine.
The reason is that TM requires enough free space to create one complete restorable system backup, plus sufficient space to create at least one additional backup of information that has changed since the previous one, plus an additional amount of "overhead" that is not easily calculated (but a very approximate guess of 15% can be considered a minimum, and a lot more than that is possible).
In other words,
Notwithstanding this clear math (the BP disc is larger than the target disc) ...
That's not all there is to it.
.... and the fact that TM should erase the older BUs to make room for the new one,
TM only does that after successful completion of a subsequent backup. You can see it do that if you were to examine its logs. You have that backup. TM will not touch it until it creates a newer one.
The salient principle to keep in mind is that TM guarantees an absolute minimum of one and only one complete, restorable system backup. More than that is "nice to have" but not guaranteed.
Bottom line: if TM says it needs more space, then it needs more space.
Thank you so much...that's was enlighitining
It is a big problem given the gigantic amount of space taken by my photo library...The biggest HD I found was 12TB...so I really don't know what to do.
I am giving a chance to Get Back Up Pro 3..let's see how it works...
Time Machine says disk full but it shouldn't. How to fix it?
