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How can I erase my phone completely and securely before selling it for someone else? I have read that secure deletion procedures used on regular hard disks are ineffective on flash drives connected through USB, due to inability to tell the embedded controller to effectively erase all of the data, or something like that.
Not sure about internal storage though (both phone storage and internal SD card), however both of this and the SD card should be erased securely, so that I can be absolutely sure that none of my private data will be available to the next user.
About the duplication status
I have checked other answers and I could not find evidence that Eraser, CCleaner or similar tools are going to wipe the SD card. In fact, I remember taking a look at a paper demonstrating that even Gutmann algorithm is not effective on flash drives. For example, here we find evidence that tools such as Eraser or CCleaner will not handle it:
You can't just overwrite a file to securely erase on a flash drive due to internal logical to physical remapping of flash blocks by the controller. When you overwrite a file, the new contents will simply go to the next free flash block. The old block will be erased later by the controller during idle/garbage collection cycles, but only if no other files are using parts of that block. Until that happens, the contents of the old file will still reside in that flash block.
The only way to securely erase a flash memory drive without a built in secure erase function is to completely overwrite the entire media to full capacity such that there are no more free flash blocks so that every block must be erased and overwritten.
Even so, I am still not sure if overwriting the entire drive would do the trick, because I remember something like flash drives have some sort of extra space used to prevent decrease of total space due to damages to specific areas of the memory during its lifetime. And this extra storage would not be normally accessible by these usual tools and procedures.
About internal storage, I am not sure if I trust Nuke My Phone either, specially because I want to erase whole phone, not automatically selected parts of it. I also do not understand well how a simple app would have such permission to wipe all of "your data" which I would assume may include not just your documents but settings from other applications, all this without rooting.
I could not find not even an explanation on the hardware architecture of internal storage, which is necessary since every kind of storage device has its own particularities and, if ever possible, their own secure erasing techniques.
So in sum, I would really appreciate documentation proving that physical deletion is indeed possible both for external SD cards, with their flash drive architecture, and internal storage, whatever architecture is used there. Thanks in advance!