Comment by dspillett

> but you'd usually get garbage data, not all zeros.

You are less likely to get garbage with an SSD in combination with a modern filesystem because of TRIM. Even if the SSD has not (yet) wiped the data, it knows that a block that is marked as unused can be retuned as a block of 0s without needing to check what is currently stored for that block.

Traditional drives had no such facility to have blocks marked as unused from their PoV, so they always performed the read and returned what they found which was most likely junk (old data from deleted files that would make sense in another context) though could also be a block of zeros (because that block hadn't been used since the drive had a full format or someone zeroed free-space).