How time machine works?

1. TM initially backs up everything that you have not "excluded" in your TM preferences. Then it backs up only incremental changes. It only looks as if everything has been backed up again on your desktop while in TM.

2. If you want it to run automatically then must be logged-on at the time TM runs. You can force a backup by right-clicking on the TM icon in the dock and then clicking on "Back Up Now".

3. TM only backs up to your external disk when you are logged-on, and hooked-up to your external disk.

+1. Does time machine back up everything i do. Or. Does it just back up everything thats changed every hour.+

Time Machine only backs up new or changed files but it uses hard links to keep track of what is currently on your hard drive that Time Machine has previously backed up. You can read about how Time Machine uses hard links in this Ars Technica review.
http://arstechnica.com/reviews/os/mac-os-x-10-5.ars/14

+For example if i received an email and then deleted it a couple of minutes afterwards would that get backed up?+

Unless the email was on your hard drive when Time Machine did its hourly backups the deleted email would not be backed up. Also, Time Machine retains the first backup of the day for its daily backup, deleting previous hourly backups. If a file was not included in the first backup of the day and was on your computer for less than a day it will be deleted when Time Machine deletes its hourly backups. After a month Time Machine retains the daily backup made on the same day as the initial backup for its weekly backup. If a file was not in that specific backup and was on your computer for less than a week it would not be retained.

+2. Im guessing it only does it every hour, so in that case does my hard drive only need to boot up every hour?+

I'm not sure I understand this question. If your Mac is not booted up there wouldn't be anything new for Time Machine to back up.

+3. If i don't use my computer for a day would time machine know that and not do anything at all that day?+

If one account is logged in Time Machine will make backups as some system files will change. I stay logged in and leave my Mini on pretty much all the time. Time Machine makes backups whether or not I am actually using the computer but the backups are quite small.

<<ime Machine retains the first backup of the day for its daily backup, deleting previous hourly backups. If a file was not included in the first backup of the day and was on your computer for less than a day it will be deleted when Time Machine deletes its hourly backups. >>

Are you sure of this? can you point me to some apple doc thats says this? I though that if you add files during the day, they get added to the daily backup file when it gets created. it wouldn't make sense any other way. Using email as an example, TM backs up at midnight or thereabouts (first daily backup). I get 300 emails during the day. Are you saying that these get discarded when the hourly backups are deleted?

+Are you sure of this? can you point me to some apple doc thats says this?+

Unfortunately, Apple documentation about Time Machine is regrettably thin and none of it seems to provide any accurate insight into what data is retained and what data is discarded. However, I've tested this twice and a couple of other people have as well. So far, no one has found that Time Machine's dailies retain data that is on the hard drive for less than a day and is not included in the first backup made for the day. I would very much encourage you to test this yourself to see if it works any differently for you. If it does it would be most helpful if you would post back your results.

+I get 300 emails during the day. Are you saying that these get discarded when the hourly backups are deleted?+

If the emails are on your hard drive for less than a day and they are not present on your hard drive when Time Machine makes its first backup of the day they will not be retained in the daily backups.

Basically, if you want Time Machine to keep a file in its daily backups without worrying about which backup it is or is not included in keep the file on your hard drive for over a day. Similarly, if you want to sure the file survives in the weeklies leave it on the drive for over one week.

I too have found this to be true. I've added file(s), forced a backup to preserve them, deleted the file(s) forced a backup and confirmed the files exist in backup history. 3 days later with only a daily backup present for that day the file(s) were nowhere to be found in backup history.

I've just used Apple's feedback form to register this and included my test case. We all need to report this - the more people report it the more likely Apple is to fix it.

My guess is that the current operation is quite simple to implement. If Apple were to create a daily backup from the hourly backups by performing a mathematical union of all hourly files, than the algorithm would be more complex to implement (but do-able), however the time factor to do this might be an issue. Who knows.

But, as many have discovered, TM is not a safe "archive" application. Its one dependable feature is restoring a full system after a crash or system problem and having the restored version relatively up-to-date. (That's a good reason for letting TM do hourly backups instead of overriding the system by doing them once every day or some similar thing.) I've already restored two Macs using the install disk and TM, and have had the process work as expected.

I agree that it's probably a lot easier to implement. You made me start thinking about the problems of trying to union all backups during the day and you're right Apple would have to do something with files that had multiple versions during the day... Really out of the scope of TM.

I think the best thing Apple could do then would be to allow users to modify the default removal schedule. For my system I could easily allow hourly backups to be kept since not that much changes intra-day but when it does change I'd like to be able to go back to that point - especially if I've specifically requested at backup.

Anyway, my point about giving feedback still applies. The more of us that give feedback the better.