Secure Empty Trash not 100% safe

Question

Recently I use finder to enter directly into Firefox's cache files and move all the files into the Trash. Next I use Secure Empty Trash.
After that, I used Data Rescue II to see what i can find.
I am very surprised to find most of the images that i have securely deleted.
I know the best way is to use Disk Utility's erase free space function, but if Secure Empty Trash is going to write the data 35 times over, and still can't get the job done.
Then using Erase free space's 35 times write-over isn't completely safe too?

Answer 1

Normally when a file is deleted insecurely, the data isn't deleted at all - the only thing that is removed is the reference to the data in the file catalogue. Overwriting a file even once (let alone 35 times) should prevent recovery by programmes like the one you mentioned that are designed to detect unlinked files and create new catalogue entries for them. Given the above, a few possibilities come to mind that could account for your observations.
One is that the files you recovered were not the ones you thought you deleted. For example, working with "TextEdit.app", any time a previously saved document is modified, an invisible "autosave" copy is created on the hard drive, intended to preserve any modifications in case the programme or computer crashes. This copy is removed insecurely once the changes are manually saved. If the original is then securely erased, it will be unrecoverable. However, data recovery programmes can easily recover the remnants of the multiple intermediate "autosave" files. I have no idea how "FireFox" works with cache files, but perhaps what you recovered was remnants from previous times you visited the sites that were cached in the files you securely deleted.
A second, less likely possibility is that your files were recoverable due to a design flaw introduced in Leopard's version of "Secure Empty Trash" (I haven't tested recently so I don't know if the issue has since been addressed). It is possible to configure it so that files are not securely deleted, even when selecting the option. However, such a configuration would not be a random occurrence - basically, a user would have to set it themselves, although malware or a malicious person with access to your account could do the same...
A third possibility, perhaps related to the second, is that in Leopard, regardless of whether the over-writing stage of "secure empty trash" succeeds or fails for any reason, the process will still continue to the unlinking stage, in effect deleting insecurely. Prior to Leopard, if over-writing failed, the file would not be unlinked, which had at least two benefits: i) since the file was never insecurely deleted, it would not be necessary to initiate a time consuming "erase free space" procedure to ensure that the data was destroyed; and ii) the very presence of the file serves as feedback to the user that the procedure has failed, unlike in Leopard where it fails silently.
Either way, using "erase free space" shouldn't be affected by these issues. If the file has been deleted (i.e. copies do not still exist in a hidden cache file somewhere), then overwriting "free space" should prevent basic recovery software from restoring it, assuming the procedure is allowed to run to completion.