I changed my startup drive to an external drive temporarily, started up from that drive and then reverted to the original internal drive and restarted. TM now wants to start a new backup. The old backup file is sitting on the backup disk ready to go but TM does not see it. How can I get the previous backup file to be available inside TM.
Mac Pro, macOS High Sierra (10.13.2)
This is actually pretty standard.
You will have to wait for the first backup to take place .. then, and only then, will Time Machine check the disk for previous backups, discover it, test it, and continue with a standard incremental backup.
Anytime you change over something in a major way.. this is how TM behaves.
That first backup be warned can take a long time.. as the entire backup will be checked.
This is actually pretty standard.
You will have to wait for the first backup to take place .. then, and only then, will Time Machine check the disk for previous backups, discover it, test it, and continue with a standard incremental backup.
Anytime you change over something in a major way.. this is how TM behaves.
That first backup be warned can take a long time.. as the entire backup will be checked.
Thanks. Very useful information. However there remains a problem. My backup disk is 3TB. The old backup file is 1.52 TB and so TM reports that there is not enough spare space to do a new backup. I am going to try excluding some material from a new backup to see if that will get around the problem. If it does, hopefully your prediction will come true for me. I suppose that I could then delete the new backup after TM has decided to continue with incremental updates of the old backup.
OK.. that is pain.. it would normally have just worked.. but the situation where you have too little space available was not taken into account.
You can attempt to inherit the backup.
https://simon.heimlicher.com/articles/2012/07/10/time-machine-inherit-backup-usi ng-tmutil
Using command line and attempting to do this kind of thing.. as you know.. is fraught with danger.. USE AT OWN RISK.
Before I got around to trying tmutil TM decided to do another backup and this time seems to have wiped the historic backup file so there is no opportunity to use the command line suggestion. Unfortunate. At least I am now aware of a TM issue in the event of starting up from other drives for whatever reason when combined with disk size constraints.
It should have warned you that it was going to delete the old backup .. although we note of late Apple tends to hide these kinds of notices as unimportant.
My recommendation at this stage is to give Time Machine a long rest.. and start using a third party backup software like Carbon Copy Cloner.. it at least does what you tell it.. and is much easier to control.. plus it will never wipe out its own backup for extraneous reasons.
I have Super Duper making a nightly backup of my internal drive to another external drive but that backup is only a snapshot that is replaced every night whereas the attraction of TM is the historical trail.
I am not familiar with Carbon Copy Cloner. Can it do incremental backups like TM without needing very large amounts of disk space.
CCC is faster than SuperDuper and always does incrementals.. which I would have thought superduper was also doing.. but CCC can also store history.. if you want.. but it is easy to specify limits and how extra space is used.
Anyway it free for one month usage so give it a try.. a fair number of people around here are using it.. either as secondary backup or in fact replacement to time machine.
Time machine not recognising TM backup file
