How do you know a camera card copied correctly?
You dragged the card to two drives and the progress bar finished. So the footage is safe — right? Not necessarily. “The copy finished” and “the copy is correct” are different claims, and only one of them lets you format the card. Here’s how to know the difference.
Why a finished copy isn’t proof
When software writes a file, the operating system reports success as soon as the data is handed off — often before it’s even fully on the disk, and without ever reading it back to confirm it matches the source. A dropped bit, a marginal cable, a reader that hiccups: the file looks complete and isn’t. The Finder will never tell you. This is exactly the gap offload tools exist to close.
Read-back verification, step by step
Real verification means the tool reads the copied file back off the destination disk and compares it to the source:
- Checksum the source — compute a fingerprint of each file as it’s read off the card. (Checksum verification explained.)
- Write to every destination.
- Read the copy back — crucially, with the OS cache bypassed, so you’re reading the actual disk and not a copy still sitting in memory.
- Compare the checksums. Match means bit-for-bit identical. Mismatch means catch it now.
- Record the result in an ASC MHL manifest, so it can be re-checked later without your software.
If your tool doesn’t read the copy back — if it only “verifies” the data still in memory — it isn’t really verifying the disk.
How to check it yourself
- Look for the word verified, not just “copied” or “done.” A good tool separates the two.
- Confirm the copies are on two different physical drives, not two folders on one disk.
- Keep the MHL manifest — you (or post) can re-verify the footage from it at any time.
When it’s safe to format
Only when every file is verified on at least two physical drives, with a manifest to prove it. That’s the bar Olycen holds to: it shows two numbers — copied and verified — and offers the card for formatting only once the verified count reaches every file on two drives. Until then, formatting stays off the table. See how it works or join the beta.
Keep reading: How to offload an SD card to multiple drives on Mac · Checksum verification: xxHash vs MD5 vs SHA-256 vs C4
Offload that never lies to you.
Olycen copies every card to as many drives as you want, verifies each file by reading it back, and only says the word verified once the proof is in. One-time $129, two Macs, no subscription — the launch list pays $99.