This may be a stupid question, but where does CTM store its data, and is there anyway of specifying where. I have a rather crowded system disk, but other disks are quite empty.
Hi stephm and welcome to the forums.
The snapshot data is stored on the disk it is protecting. If you choose to protect C:, then the snapshot data is stored on drive C:. The data is NOT stored as a “file” that you can see (or copy or move or delete) using Windows. The data is written to the unused space on the disk using a RAW driver that bypasses Windows.
Currently, you can’t specify what drive to store the snapshot data on, although this has been asked for in the past.
It is recommended that you have at least 10GB of free space (as reported by Windows).
Hope this helps,
Ewen 
Is there a way to determine how much disk space is used by Time Machine?
Is it reported when one does a property on the disk?
The only way to tell is to open CTM’s GUI, do a right click on a snapshot and select PROPERTIES.
Windows is absolutely unaware of the snapshot data and can’t report it. Only CTM can give this info.
Cheers,
Ewen
Interesting…how did you find that out panic?
Anyway, if Windows doesn’t know about the data couldn’t it just overwrite it by mistake?
When I noticed that a snapshot didn’t alter the amount of free space reported by Windows. 
Anyway, if Windows doesn't know about the data couldn't it just overwrite it by mistake?
CTM’s RAW disk access driver prevents Windows doing that.
Ewen
It also appears in the time line view.
The baseline size is reported as zero, which is interesting.
The other size is reported as a number on the order of 1.3 G (for a system with 125 G of which 93G is used).
I have to say the pie chart under properties is confusing, it makes it look like the baseline used 80G.
There’s quit a mess in the figures. I still can not get them fully after all the explanations. I’m really too lazy to calculate all those minuses pluses etc. and why - see the figures on a computer’s screen to calculate them!? A joke of humor
Here is a quote from a PM from egemen:
Interesting!