Copy once, verify in the background

Backing up to two drives sounds like twice the wait. Done well, it is not. The trick is how many times you read the card, and when you verify.

Read once, write to both

The slow way is to copy the card to drive A, then copy it again to drive B. That reads the card twice and adds the two times together.

The better way reads the card once and sends each chunk to both drives at the same time. Two copies then take about as long as the slower drive, not the sum of both. It also halves the reads on the card. Stow works this way.

Two copies, in the time of the slower one.

The verify is the hidden cost

A real verify reads every file a second time, off each drive, to prove it. Run that as a separate stage and you feel it as dead time at the end. The fix is to overlap it: confirm a file is written, then verify it in the background while the next files are still copying. The wait mostly disappears behind work already happening. This is the next speed gain we are building into Stow.

The hashing itself is never the problem. xxHash runs at tens of gigabytes per second, while the fastest card reads at under four. The check is free against the disk.

Watch the bus, not just the drives

Writing to two drives at once only goes faster if the two drives have room to run. Plug both into one Thunderbolt hub and they share its bandwidth, so the pair tops out around a single fast drive and the slowest sets the pace. Give each drive its own port or bus and they run in parallel for real. On Apple Silicon Macs, each port is its own bus.

When the drives do share a bus, filling the fast one first and then the slow one can be the better order. The right strategy depends on how the drives are wired, which is why measuring beats assuming.