Time Machine is NOT designed for, or capable of, having any files "manually" added to it. Time Machine is only intended to execute its own backups to the external hard drive using its proprietary software code. If you want to add any other files to the same backup hard drive, but NOT into the Time Machine backup itself(which you can't successfully do anyway), then you'll have to add another Partition to that hard drive.
I am puzzled since I found the following...
After having a terrible hard drive disaster back in college, I’ve been horribly paranoid about my backup arrangements. Mountain Lion will humor my paranoia by letting me use Time Machine to back up to several disks at once, automatically. And since OS X supports backup to networked drives, I can keep my files current in several physical places as well, without any extra work.
in an article about 10 exciting system changes in a MacWorld article from Feb 2012.
Cannot get that overlay window to appear.
FWIW, I have gotten the sense from years of reading discussion posts and articles, that the more people "get creative" with Time Machine, the more problems are encountered.
I have a strategy that has served me well since TM was first released:
1. I maintain a separate Archive drive to which I move everything that is no longer needed (such as finished work projects) or is for reference only and will no longer need editing (such as movies, application and OS installers, etc.). This drive is automatically cloned regularly so it has its own backup.
2. I use Time Machine in its basic configuration (in my case to a Time Capsule). The 500GB TC is exclusively for TM backup and has enough spcae for more than a year's worth of the approx. 250GB worth of total data on my startup drive.
This strategy provides confidence that I can recover from anything that goes wrong with my startup drive, while at the same time providing access to all "archival" type documents without that archived data taking up space in the TM backup.
On re-reading your original post, here's another option:
You can simply drag the Dropbox folder onto the Time Capsule.
If I select my TC in Shared (in sidebar) I see folder named "Time Capsule," and inside that folder (in column view) is a disk image with the name of my computer being backed up.
I can simply drag another file or folder onto "Time Capsule" and it is added to the TC's drive, but not a part of the TM backup.
Did that make sense?
What you suggest makes sense but it did not work. The file hiearchy of TC is
TC --> Data ---> TM backup image. Could not drag the Dropbox dir onto TC, I could drag it into Data Dir. The result was that TM could not see the copy of the dropbox dir on the TC inthe data dir.
The error message when I tried to drag the dir into TC was...
Good thought though... thank you.
Sorry; forgot to mention that. Time Machine cannot back up other data stored on the Time Capsule it is using for backup.
And I wasn't clear. In my setup, the directory you have with the name "Data" (essentially, the TC internal drive) is also named "Time Capsule."
You can store other data in that directory (i.e., on that TC drive), but TM cannot back it up. Although there are probably other reasons, that makes sense becuase if the TM drive failed both the original and backup data would be lost.