I have a 500 GB Maxtor HDD with some data on that I would like to access. The disk is formated with Mac OS Extended (journaled, case sensitive). I tried to erase free space (zero out) on a pretty full HD using Disk Utility. When I did it, the progress message said "unmount volume", then "erasing volume". After that, there were 500GB of free space and all my files were missing.

Under those circumstances, is there a way I can recover those files?

Try Data Rescue or File Salvage. You could also run Repair Disk with Disk Utility in case directory damage is the culprit. Disk Warrior, Drive Genius, or TechTool Pro are more powerful, however.

BTW, why did you format the drive as case sensitive? As I understand it, that can lead to problems. Do you have a Time Machine or clone backup?

Welcome to FineTunedMac. I hope that you are able to solve this problem.

jchusi has already mentioned Data Rescue and File Salvage, but he left out one additional item you will need in addition to one of those $100 utilities. An external hard drive with the same or greater capacity as your internal drive to recover the files to. In the meantime DO NOT USE YOUR COMPUTER or you risk overwriting the erased files leaving them recoverable only through the expensive services of a drive recovery firm such as DriveSavers.

Not to rub salt in your wounds, but this is a principle reason why you should always have a good backup system such as Time Capsule.

First and foremost stop any writing on the disk until you have recovered your data. Also, no matter what file restoration application you use, never write the restored data to the drive you're restoring from. That could potentially destroy data you want to restore.

After that you can use OS X disk utils or third party programs to attempt to recovery files without the worry of destroying any data.If none of the cheaper options work, you might try [link removed], it should get the job done.