Understanding Flash memory

Hi. I'm trying to get started in using Flash memory (SD cards) in projects. My understanding was that flash can not be written to unless it's first erased; and that, unlike magnetic media, deleting a file from flash actually overwrites the data with 1111s. However, some experiments with a SD card reader on a PC and a drive imager show that even after deletion, the data bytes are still there, and that new files seem to overwrite them.

Can anyone clarify whether Flash is directly overwriteable, or needs an erase command first? What happens when you delete a file from a Flash memory card?

What's the best way to get started with Flash hardware in projects? Any good guides?

I believe what you are seeing is just the directory redirect writing. When a new block is needed for new data there is a block management decision on if and where to clear an old block to all 1's. This way time is not wasted when there are available cleared block to save data.

Thanks. Is there anyway to access the physical level, underneath the virtualization.

Modern flash drives include wear levelling to mitigate wear from repeat write cycles.

Wear levelling - Wikipedia, the free encyclopedia