Q: Why is my hard drive constantly writing and using up all available space ?

The activities in progress as shown in Activity Monitor, or system logs from Console,

may yield some clues on what may be running in the background. - Do you have a

Time Machine backup saving to same hard disk drive your system is installed on?

That could use up space and make the system itself slow, also hasten its demise.

If there is something, the evidence should appear there. You may have to start

the computer in SafeBoot to help determine what settings or items to change or

to consider removal. There also may be adware, or other items on the OS X that

do no good; some antivirus scan software can waste system resources for no use.

If the computer shipped with install-restore DVD and has optical drive, and if it still

is running a similar OS X, you could attempt to check & maybe repair the hard drive

or see what is going on by use of Utilities from the boot installer, to check the HDD.

And, if you have a later system and it uses Recovery, you could boot into that to see

some use of OS X Utilities to attempt to fix the issue.

But whatever is running, you'll have to determine. And turn off what is using extra

processor capacity and writing to the HDD. Could be the drive is way full and the

OS X is trying to survive on swap file sharing (read-write) almost constantly.

So that may be somewhere to start.

Good luck & happy computing!

Late to the party, but I had the same problem.

I told spotlight not to index ANYTHING. Didn't work.

I ran open snoop and uninstalled any programs that had files that were being accessed. But it just switched to different files. Didn't work.

The solution for me was to go into the Java system preferences and under the General Tab>Delete Files... check all the boxes to delete everything.

Hope this helps someone.