Getting inside Time Machine's head

My remedial understanding of TM is that it backs up everything but the core OS files (i.e., you have to reinstall OSX and then run TM, but it will also restore your OS preferences/settings). It seems that TM can take quite a bit of time for the initial backup (which makes sense), then subsequent sessions run very quickly, as it's only backing up changed files...in other words, an incremental backup of sorts. I say "of sorts" because one can choose any day in TM's inventory and it will restore your system to the exact overall state of that day. To do so, it must be effectively assembling a "full" backup every time, hence my questions:

In my limited knowledge, I would guess that TM takes the last backup set and "brings it forward", overlaying changed files on top. This is why the incremental backups run quickly - most of the data is already there - and also seems the practical way to allow the option of a complete restore to any point in the dataset, without running a full backup routine every time. Correct?

I want to start backing up two Macs to a single external drive using TM, and swap the drive out with another regularly to always have one off-site (rotating the two monthly or so). The drive is currently full, and I assume (perhaps incorrectly) that TM for Machine B will not delete old backups for Machine A if the disk is already full. So, can I choose a range of dates for Machine A on my backup disk and delete them, without destroying TM's ability to restore subsequent dates? I suspect so, since it automatically deletes old backups when space is required. That intuitively tells me that TM is effectively creating a "full" backup with every run, as an algorithm to selectively delete only the data it would not need to assemble a full backup set on demand would be exceedingly complex.

Will TM automatically create separate ID'd datasets on the drive, using a single partition? I also assume each machine's TM runs will manage their respective datasets for disk space requirements?

Thanks in advance for any insights.

Tom

Yes, TM will create separate files to save the data in based on the machine name and each machine will use only its specific file.

TM uses the MAC of the HD - u can see it here;


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and if you want it to add to a previous sparsebundle then selecting 'Enter Time Machine should give you this window;


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Thanks for that.... It uses the name and MAC, I knew I was was forgetting something.