Before you format a memory card, read this. Formatting is the right thing for a new card and the wrong thing for one you want the pictures from.

A quick format is what nearly every device and every default dialogue runs. It writes a new boot record, new empty allocation tables and a new empty root directory at the start of the card, a few kilobytes in all, and leaves the data area where the photographs and video sit untouched. The SD Association's manual for its own formatter says that after a quick format the data still remains in the card and may be retrieved by using a data recovery tool. A camera's format is a quick format on nearly every body. A phone's reformat of a portable card is a quick format. A Switch's format of a card is, as far as the bench sees, a quick format. None of them erases the pictures; all of them erase the map to the pictures and start the card writing over them with the next file saved.

A full format is the Windows dialogue with the Quick Format box unticked. Microsoft's knowledge base article 941961 documents that from Windows Vista onward the format command writes zeros to the whole disk when a full format is performed. After it, nothing is on the card and nothing can be recovered by anyone. macOS's Disk Utility erases a card by writing new structures and, with its security options, by overwriting; its default erase is closer to a quick format, and its secure options are not.

The SD Association's formatter, which the Association says it strongly recommends over operating-system tools for SD, SDHC, SDXC and SDUC cards, offers a quick format, which leaves the data, and an overwrite format, which its manual says erases all data by overwriting the user data area completely. It also lays the file system out the way the card's controller expects, which is why devices behave better on cards formatted with it than on cards formatted on a PC.

In-camera and in-device formats are quick formats on nearly every body and device. A few camera bodies offer a deeper option, Canon's low-level format among them, which goes further than the quick one; what each body's deeper option writes is not something to discover on a card with the pictures on it. Dashcams and home cameras, Nextbase, Garmin, Tapo, Ring and the rest, format cards to their own layouts when a card goes in and ask again when a card errors; the first is fine on a new card and the second is a worn card asking to be retired, with its footage readable until the format runs. A Switch's format at the end of its error page is the same quick format with Nintendo's own warning beside it that a formatted card cannot be recovered by the console.

When to format.

A new card, in the device that will use it. A card moving from one camera to another, after its pictures are copied off. A card going into a dashcam, a doorbell or a trail camera for the first time. A card that has been recovered and is being put back to work, if it was not the card's own fault. A card you have backed up and want clean. In every one of those there is nothing on the card to lose, and the format makes it behave.

When to stop.

When the device is asking you to format because it cannot read the card, and there is anything on the card you want. That is a symptom, not an instruction: the camera's card error, the computer's you need to format the disk, the phone's SD card is corrupted, the Switch's 2016-0247, the GoPro's SD ERR, the dashcam's please format are all the same message, that the file tables cannot be read and the pictures are behind them. Say no, take the card out, and have the pictures recovered first; format it afterwards or, if it was the card's fault, replace it. The pages under what it is saying take each message in turn.

When it is already done.

A quick format that was accepted leaves the pictures where they were, and the card can be recovered provided nothing has been written to it since; stop using it now, because every new file is written into the space the old ones occupy. A full format on Windows cannot be undone and the bench will say so at the free look, with nothing to pay. If you are not sure which was run, the Windows dialogue's Quick Format box is ticked by default, and a format that took seconds was a quick one. Formatted, or deleted by mistake says what comes back.