We tried it on a small test drive set up like a USB stick: we deleted a test file named photo, then read the whole drive byte by byte. Every one of its 98,304 bytes was still there, in the same place.

Deleting doesn't erase a file. A drive keeps two things for every file: its bytes, in blocks, and a record with its name, its size and where its blocks start. Deleting changes the record. On our test drive one letter changed: the P of PHOTO became 0xE5, which Microsoft's FAT32 specification reads as "the directory entry is free". Unix did the same in 1974; in Ritchie and Thompson's words, the file's disk blocks "are freed". Freed, not wiped: the bytes stay until something new is written over them. That's also why deleting is quick: on a laptop, deleting 1 GB took 0.29 seconds and writing it 1.76 (medians of five runs).

What to do: before you sell or give away a phone or a laptop, don't just delete. Erase it. Apple's erase "obliterates all the keys" the data was locked with, so every leftover byte is unreadable. On Windows, Reset this PC with Clean data "makes it harder for other people to recover files you've removed".

The stick, the blocks and the record are our own simplified drawings.

What's the oldest phone in your drawer?

Sources: our own measurements on a laptop, 2026; Microsoft, FAT32 File System Specification 1.03 (2000); D. M. Ritchie and K. Thompson, The UNIX Time-Sharing System, Communications of the ACM, 1974; Apple Platform Deployment, Erase Apple devices (2023); Apple Platform Security, Data Protection in Apple devices (2021); Microsoft Support, Reset your PC.
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