Folder Sync and Backup

About syncing two folders

robocopy src dst /MIR does this job well, and it is the reason nobody should type it from memory. /MIR mirrors, and mirroring means it deletes from the destination everything that is not in the source. Get the two paths the wrong way round and a backup drive empties in seconds. There is no preview and no undo.

This tool does the same work in the opposite order. Both folders are compared first, every file is put in a bucket, and the counts and sizes are on screen before any button does anything.

How to use it

• Choose the source folder — the one with the good copy
• Choose the destination — where things should end up. Your browser will ask permission to write there
• Leave Add new and update changed selected. It never deletes anything
• Read the lists: what is new, what changed, and what is only in the destination
• Press Sync now — or take a robocopy or rsync script instead

The four buckets

• New — in the source, missing from the destination. These get copied
• Changed — in both, but different. These get overwritten
• Only in the destination — not in the source. Left alone, unless you choose to mirror
• Already identical — skipped, and they cost nothing

What counts as "changed"

This is the option worth understanding.

• The size differs — the fastest, and it already catches almost every real edit
• The size or the date differs — also catches an edit that happened to keep the same size. This is the default, and what robocopy does
• The contents differ — a SHA-256 of both copies. The only answer that is certain, and the only one that reads every byte of both folders, so it is slow on purpose

A file whose bytes match but whose timestamp does not is identical under contents and changed under date. That is not a bug in either mode — it is the difference between them, which is why the report says which one produced the answer. Timestamps that differ by a second or two are treated as the same, because that is what copying between two different filesystems does to a file that was copied perfectly.

Mirroring, and why it is not the default

Mirror also deletes from the destination whatever is not in the source. It is the right choice for a true backup drive that should be an exact copy — and the wrong choice roughly every other time.

So it is a separate option, the files it would delete are listed by name, the warning names both folders, and a tick box has to be ticked before the button will work. The tick clears itself every time the list changes, because consent given for one set of files is not consent for a different one.

Deletions also happen last, after all the copying. A failure part way through can never leave the destination emptier than it started.

Copying goes one way only

Source to destination, never back. A two-way sync has to decide which side wins when both copies changed, and a tool that guesses that is a tool that loses work. The source folder here is opened read-only and is never modified.

What people use it for

• Backing up Documents or Photos to an external drive
• Keeping a USB stick up to date with a working folder
• Copying a project folder to a second machine without re-copying gigabytes
• Refreshing a website folder with only the files that changed
• Checking a backup actually matches before trusting it, using contents mode
• Merging a folder from an old laptop into the one on the new machine

Good to know

• Works on Windows, macOS, Linux and ChromeOS. Mobile browsers cannot hand a folder to a web page
• Nothing is uploaded. Both folders are read in the page, and only the destination is ever written to
• Firefox and Safari can compare and give you a robocopy or rsync script, but cannot write the files here — Chrome, Edge and Opera can
• Big files are streamed straight to disk, so a 4 GB file never has to fit in memory
• The destination is re-read after a sync, so the list you are left looking at is the truth
• Contents mode reads every byte of both folders. On a large pair, expect it to take a while
• Each list shows the first 250 files
• The script has one robocopy or rsync line rather than one line per file — and its mirror line is switched off until you turn it on

Related tools

• Compare Two Folders — the same comparison, with no copying at all
• Duplicate File Finder — clean up before backing up
• Folder Size Analyzer — see what is filling the drive
• Files Changed Since — what moved since your last backup
• Bulk Delete Files — clear out the destination by rule instead