Verified copying to an external drive
CardSync backs up to an external drive — typically an SSD over USB-C — and verifies every file it writes. The source can be a folder, such as a memory card, or an album from your photo library.
Keep CardSync on screen while it copies. iOS suspends apps sent to the background and does not keep external volumes available to them. The app prevents the screen from locking; if you leave, the copy stops cleanly and resumes where it left off next time.
The source and the destination are two folders, picked in the Files app. Anything iOS exposes there works, in both directions:
Nothing requires starting from a card: SSD to SSD, iCloud to SSD, card to iCloud all work the same way. When a file is not yet present on the device — common with an online service or with some card readers — CardSync waits until it is genuinely available before reading it, rather than copying an empty placeholder.
Files are read and written in small successive blocks, never loaded whole into memory: a 4 GB video uses no more memory than a photo — measured at about 90 MB for a verified 4 GB copy. File size is therefore not a limit.
For copies that take a while, what matters is resuming. A file only appears under its final name once complete and verified, and the next analysis compares both sides: running it again after an interruption — screen left, drive unplugged, battery — copies only what is genuinely missing. On a set of several tens of thousands of files that analysis remains a matter of seconds, and files already verified are never redone.
Every file is first written under a temporary name, then put in place in one move, once complete and verified. A file already present at the destination is never deleted before its replacement is ready, so an interrupted copy never leaves a truncated file behind.
For a folder source, the SHA-256 digest of the file written is compared with the original, itself read a second time. If the two reads of the original differ, the card or the card reader is at fault, and the app says so instead of copying corrupted data.
For a photo library source, the comparison is made against the digest computed during the export: the system exposes no re-readable original file.
Pick an album, a smart album or the whole library, with an optional "from a date" filter. CardSync exports the original file and, if you want, the Live Photo video, the edited version, and a readable metadata sidecar holding date, location, dimensions, favourite and album.
Items no longer stored on the device are fetched from iCloud during the copy. That can take a while and use data: the option can be turned off in the sync's settings.
An encrypted drive re-locks every time it is unplugged. Tap the Open button offered in the error message: iOS then shows its password prompt, and the analysis restarts on its own. Try again is for when you have already unlocked the drive elsewhere.
Many external volumes cannot report their available space. CardSync would rather show nothing than a wrong figure, and this never prevents a copy from starting.
Some drives refuse to record modification dates. The app reports this in the summary. In the sync's settings, choose the Size only comparison: it does not depend on dates.
What was written did not match the original; it was deleted, and the file already present at the destination was left untouched. This usually points to a failing cable, card reader or drive. Try another cable or another reader.
Question, problem or suggestion: laurent.meurisse@mac.com.
For a copying problem, please mention the drive model, its format (exFAT, APFS…), and the exact message shown in the app's log.