How copies are checked: Fast, Thorough or Off

Every folder move checks the copy before your original is removed. The Verification level decides how carefully. Fast is the default and right for almost everyone; Thorough is for a drive you don't fully trust; Off is the quickest and checks the least.

Where to set it

In Data Parking, click the gear in the middle column to open Data Parking settings, then choose under Verification level. The setting applies to every folder move, including moves from Storage Inspector and moves back.

The three levels

Fast works because a correct copy has exactly the same size, date and permissions as the original. If any file's details differ, Storage Studio reads that one file on both sides and accepts it only if the contents match.

Thorough reads everything twice over, once from your Mac and once from the drive, so it catches damage that the drive itself doesn't report. It takes much longer on big folders.

Off skips the file-by-file comparison. Storage Studio still compares the total size of the copy with the original before removing anything, so a copy that came up short is caught — but a damaged file of the right size would not be.

Merge is checked a little differently

With Merge, the destination is allowed to hold files that you didn't move. Fast ignores those extras and still compares every file you did move. Thorough can't compare the whole folder in that case, so for a Merge it relies on the total-size check — which makes Fast the more careful choice for merges.

%% caption: What happens after the copy, at each verification level
flowchart TD
  A[Copy finished] --> B{Verification level}
  B -->|Fast| C[Compare size, date and permissions of every file]
  C --> D{Any differences?}
  D -->|Yes| E[Read those files on both sides]
  D -->|No| G
  E -->|Contents match| G[Check total size]
  E -->|Contents differ| X[Stop - original kept]
  B -->|Thorough| F[Read every byte on both sides]
  F -->|Match| G
  F -->|Differ| X
  B -->|Off| G
  G -->|OK| H[Remove the original or leave shortcuts]
  G -->|Too small| X

If a check fails

The move stops and your original stays exactly where it was. The Moves list says, in plain English, that the copy didn't match the original closely enough to be sure it was complete. Try again; if it keeps happening, something may be changing the folder while it's copied — see When an app is holding a folder open — or the drive may need attention, see Checking your drive's health and setup.

While a check runs, the Moves list shows progress such as Checking every file arrived intact — reading the copy on your drive.

Parallel transfers

The same settings popover has Parallel transfers: how many copies run at once, from 1 to 16, default 5. A lower number suits a slower drive. Changing it doesn't interrupt copies already running; the next one picks it up.

See also: Moving folders safely, Verified copy, What a move keeps.