Backup vs overflow recording
Backup recording writes every photo to both memory cards at once; overflow fills one card, then continues on the other, giving more space but no second copy.
Cameras with dual card slots ask how to use the second card. Backup (sometimes called mirror or simultaneous recording) writes each frame to both cards, so a failed card costs nothing. Overflow (auto-switch, continuous or sequential recording) uses the second card only when the first is full, doubling the space but keeping just one copy of each frame.
The choice is a trade between safety and capacity.
How it works
In backup mode the camera writes the same file to both cards; the shot isn't finished until the slower card has it, which can shorten bursts. In overflow mode the camera writes to one card at a time and switches automatically. A third option, separate recording, writes RAW to one card and JPEG to the other — each frame exists once in each format, not twice in the same format.
An example: a card that dies mid-ceremony
A photographer's SD card throws an error halfway through a ceremony. Because the camera was recording in backup mode to a CFexpress card as well, every frame is safe on the other card. On overflow, the first half of the ceremony would have depended on recovery software.
Card modes and CloudTether
Tethering adds a third copy that leaves the camera: CloudTether copies each shot to the phone within seconds and uploads it to your destinations, so even losing the whole camera doesn't lose the shoot — see never lose a shoot to a lost card. It doesn't replace backup recording, though. With JPEG only on, the RAW files exist only on the card until you get home, so backup mode keeps them safe. On a single-slot camera, delivering full-size files to Dropbox or Google Drive is your backup.
Which to choose
Weddings, events and anything you can't reshoot: backup. Long sports or wildlife days where space matters more: overflow, with frequent offloads. When clients need JPEGs and you keep RAW: separate, ideally with tethering providing the extra copy.
Tips and common mistakes
- Set the recording mode in the camera before the job — it's easy to forget after a reset.
- Buy two cards of similar speed for backup mode.
- Offload and verify both cards before formatting either.
Put it into practice
CloudTether tethers your camera to your phone and delivers every shot to the cloud, live as you shoot — no laptop.