Time Machine is starting over on secondary internal drive!

[stiligFox]

Can someone help me with understanding why Time Machine is starting over on backing a drive?

Here's my set up -

500GB SSD - main OS with about 450GB of data
2TB HDD - secondary drive for bulk storage with 998GB of data.

They are both installed internally into the computer. What happened I think is that one day I removed the 2TB HDD to access it's data from another Mac, leaving the original Mac without that drive when it did a backup. Now that I have put that drive back in, it is proceeding to back up 998GB of data as if that drive is completely new, even though nothing has changed on the disk. Does anyone know why this is happening? It's going to completely fill my back up drive and that's a whole lot of reading and writing of data for no good reason. Is there a way to prevent this next time, besides not turning on the main machine while the disk is missing?

 

[Honza1]

It may not be that bad yet. TM may (should) figure out that it has those files already and just link to them, so the size may be small in the end. It figures that out during the backup, not ahead. So the reported needed size may be wayyy overstated.

It is difficult to say, what happened from your description. IT is possible that somehow the drive is assigned different identifier and TM thinks it is a new disk. If you plugged it into a different Mac, that may have changed something on the disk. I am not sure what happens with internal drives when they move around this way, but I would not expected that to happen.

In any case, TM evaluates need to do new full backup routinely and if it thinks it need to do so, it will force it. Typically that is due to error in TM records which cannot be fixed, though. I do not think this is your case here.

 

[stiligFox]

It may not be that bad yet...

Thanks for the reply! It so far is at about 173 of 998GBs after an hour, so I guess I'll keep an eye on it. But you're right, I wouldn't have thought the second Mac would have changed something crucial like that...

This is speculation as I can't quite remember, but I did clear all the .ds_store files from my secondary Mac; the thing is I can't remember if I had the drive connected at that time or not. Could that be it? As far as I know .ds_store only controls how the folder looks (which view, what size and screen position) and wouldn't affect backups.

The other thing I'm wondering is that since my main Mac did a backup without the drive connected once, it assumed I deleted all the files in that snapshot, and since the next snapshot suddenly sees 998GB worth of data that wasn't in the previous snapshot (even though they were there in the snapshot before that one), then Time Machine just marked them all as new files. I would've thought that Time Machine would be able to handle drives being removed and re-added though?

I'm baffled, really hope this doesn't happen again! I like being able to pull a drive out of my Mac to share data with other Macs quickly, but not if it causes this much headache! Thanks again

 

[NoBoMac]

Is there a way to prevent this next time, besides not turning on the main machine while the disk is missing?

Not clear if tried this, but, should be able to exclude the drive from TM when not in place. Or at least just tuen off TM.

But, think you nailed it why it happened. What might have happened is TM did its thing and noticed all these files are no longer there, so, all snapshots since then no longer have the folders or files in it. Plug the drive back in, now you have all these "new" files that are not in the previous snapshot, so need to copy it back.

As @Honza1 said, hopefully, TM will be smart enough to know has an old copy of it and will make links.

However, the erased DS store files being erased would trigger a backup: TM looks at modification date on folders to determine what it needs to be backed up. Do lots of pruning, lots of folders are now "modified".