Basics of File Recovery
Files in Trash
If you simply put files in the Trash you can restore them by opening the Trash (left-click on the Trash icon) and drag the files from the Trash to your Desktop or other desired location. OS X also provides a short-cut to undo the last item moved to the Trash -press COMMAND-Z.
If you empty the Trash the files are gone. If a program does an immediate delete rather than moving files to the Trash, then the files are gone. Recovery is possible but you must not allow any additional writes to the hard drive - shut it down. When files are deleted only the directory entries, not the files themselves, is modified. The space occupied by the files has been returned to the system as available for storage, but the files are still on the drive. Writing to the drive will then eventually overwrite the space once occupied by the deleted files in which case the files are lost permanently. Also if you save a file over an existing file of the same name, then the old file is overwritten and cannot be recovered.
General File Recovery
If you stop using the drive it's possible to recover deleted files that have not been overwritten by using recovery software such as Data Rescue II, File Salvage or TechTool Pro. Each of the preceding come on bootable CDs to enable usage without risk of writing more data to the hard drive. Two free alternatives are Disk Drill and TestDisk. Look for them and demos at MacUpdate or CNET Downloads.
The longer the hard drive remains in use and data are written to it, the greater the risk your deleted files will be overwritten.
Also visit The XLab FAQs and read the FAQ on Data Recovery.
What Kappy says also you can install OS X on a external drive, boot from it and install undelete software (hard drives only) to recover data.
If you have Filevault or used Secure Empty Trash, or Zero Erase Free Space in Disk Utility or waited too long, or wrote too much to the drive, then forget it.
However if your not technically inclined, you can shut down the computer (asap) and take it to a local PC/Mac repair shop and they will image the drive to a external drive, which you can later pick through and recovery your deleted data.
However again, if the above conditions apply then it's too late.
In order to recover deleted files form emptied trash , you can have a try of the following steps:
1. Choose Lost File Recovery mode
No matter data are deleted by "Command + Delete" or emptied from the Trash, "Lost File Recovery" can undelete files from a partition or removable media. This recovery option is priority, for it takes you only a few minutes to retrieve deleted files from Mac Trash.
If you can't get deleted file in this mode, you can try "Raw Recovery" later, which can recover piles of files including those lost long time ago, more time consumed accordingly.
2. Scan the location of Trash in Mac
Select the logical drive where you deleted your files, if you have more than one drives on your Mac. Then click on "Scan" to search your deleted files.
3. Preview and recover files from Mac Trash
When finishing the scan, all files you deleted are listed with the original path, no matter you deleted them directly from Library, desktop, or Trash, etc. Undoubtedly, files you emptied from Trash are in the "Trash" folder. Or you can search the file's name in the search bar on the top to retrieve it fast.
Before final recovery, you can preview the file to make sure the files or quantity. Common image formats, documents and archives are allowed to preview directly.
4. Save the recovered files on another partition or disk
When checking the file for recovery, click on "Recover" to retrieve them.
What you should keep in mind is: don't save the recovered files on the same place where you lost them, in case you overwrite them. If you don't find all the files, the deleted files will be overwritten for your new files' storage.
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