Data Transfers · Copy, Move and Sync
Why does running the same transfer again finish so much faster?
Short answer: Repeat runs only send files that are missing or changed — everything that already matches is skipped. That's also how transfers carry on after a restart.
Because only what's missing or changed is sent. Copy and the sync operations compare the two sides and skip files that already match, so a repeat run of a large folder where little has changed mostly just checks and finishes.
The same rule is what lets a transfer pick up after a restart: when Storage Studio relaunches, a transfer that was running goes back into the queue and carries on, sending only what isn't there yet.
One exception: a single file that was interrupted part-way is sent again from the start, unless it's a large download coming to your Mac in pieces, which resumes where it stopped.
Related: Large downloads that resume