where is the Mac Operating System stored on internal boot drive??

Why don't you simply erase your external drive and install a fresh copy of OS X? The operating system lives in many different places on the hard drive hierarchy. The only things you can safely delete would be those in your Home folder - documents, movies, music, pictures - except in the /Home/Library/ folder.

You can delete anything in the Applications folder you don't need as well as anything in your user's Home folder. Don't mess with anything else unless you know exactly what you're doing which, and I mean no insult, it doesn't really sound like you do.

And go out and get yourself a new set of Mac OS X install disks, either by ordering a new set of the original disks from your Mac Pro from Apple tech support or by buying a new retail copy of Mac OS X 10.5. You may well need those disks at some point. Having them now would make doing what you want a lot easier.

That's a tricky question because the default installation puts files in several places.
/System is the obvious one.
/Library you have to keep too.
But there are also many invisible folders at the root level.

You're also not clear on what you want to do with this boot drive. If you want to not only boot your system but run some of Apple's utilities or apps, then you cannot delete
/Applications
or
/Applications/Utilities
and some things won't work right if you delete
/Users

So what is it you're trying to accomplish?

The bulk of it is in /System/ and /Library/ folders as well as some in the /Home/Library/ folder. What you can delete from the latter really depends on what you want to preserve. If you delete third-party applications as suggested, then you can delete /Home/Library/Applications Support/ items related to the deleted applications as well as preference files for them in the /Home/Library/Preferences/ folder.

If I read your intent, you plan on replacing the machine's internal HD with the SSD one. If so, then I strongly suggest you get a larger SSD. If you don't have the disks that originally came with the machine, call 800-MY-APPLE and order replacement ones. You'll need the machine's serial #. They're usually available for S&H ($20-30 USD).

Most of that is likely in your Home folder.

My /Library/ folder is just under 3 GBs and my /System/ folder is 3.74 GBs. My /Home/Library/ folder is 2.8 GBs but includes Preferences, all my Application Support folders, Address Book data, Calendars' data, and and my Mail folder. Mail accounts for .9 GBs and Applications Support about 1 GB.

A "full" installation of OS X including the basic applications and all the printer drivers maxes at about 10 or 11 GBs when installed from the Installer DVD. Unless you are trying to save a lot of personal data in your Home folder you surely wouldn't need more than 7-9 GBs.

Then, install the OS as noted and use the same username/password combo when you get to that stage , then the setup assistant to transfer only your apps. That should do it. Sorry, I misread you machine as a laptop and not a Mac Pro.

awsteele wrote:
can't because I don't have installation cds anymore.

You should contact Apple and get replacement disks for a nominal fee. Without them, there is very little you can do to service your system.