Check That a Backup Matches the Original Folder

To check that a backup matches the original folder, compare the content of every file, not just names, sizes or dates. On Mac or Linux, diff -rq original backup does that and prints nothing when the folders match. On Windows, a short PowerShell snippet does the same. Before that, a 10-second check catches the most common problem: count the files in both folders. A missing file changes the count. A damaged file doesn't, and neither does a file whose size and date look right but whose content is wrong. Below are three levels of checking, each tested on a real backup, plus what zip files and USB sticks silently leave out.

Our test backup

We backed up a Photos folder with seven files in it, plus a few things that often go wrong:

• three photos, a notes.txt and a hidden .album file
• an empty folder, 2026/Empty-album
• cover.jpg, a symbolic link (a shortcut) to one of the photos
• Private/locked.txt, a file the user didn't have permission to read
• Plan: day 1?.txt, a name with a colon and a question mark

We copied it with cp -r Photos Backup on Ubuntu 26.04. cp printed cannot open 'Photos/Private/locked.txt' for reading: Permission denied and carried on. In a file manager, a message like that is easy to miss.

Level 1: count the files

On Linux, count the files and add up their sizes in each folder:

find Photos -type f | wc -l
find Photos -type f -printf '%s\n' | awk '{s+=$1} END {print s}'

Our original had 7 files and 606,052 bytes. The backup had 6 files and 606,045 bytes, so something was missing. On a Mac, the first command works as is, and Finder's Get Info shows the size in bytes. On Windows, run this in PowerShell, which prints the number of files and their total size:

Get-ChildItem 'C:\Users\you\Pictures\Photos' -Recurse -File -Force | Measure-Object Length -Sum

This check takes seconds, even for thousands of files. It doesn't tell you which file is missing, and it can't see damage that doesn't change the size.

Level 2: compare every file

diff -rq Photos Backup

Only in Photos/Private: locked.txt

That names the missing file. diff reads every file in both folders, so it is slower, but it is the check to run before you delete the original. It also follows the symbolic link and compares the photo it points to. On Windows, use the PowerShell snippet in built-in ways to compare two folders, which compares a fingerprint of each file's content.

Why not compare size and date? Many tools do, including rsync without -c and our own Folder Compare. We changed one byte in the middle of a backed-up photo and set its date back, which is what silent damage on a failing drive looks like. The size and date still matched the original, so a size-and-date comparison reported no difference. diff -rq, rsync -rcn and a checksum list all reported the damaged photo. The opposite also happens: a fresh copy with new dates looks completely different to a size-and-date check, even when every byte is right.

Level 3: keep a checksum list with the backup

A checksum is a fingerprint of a file's content. Save one for every file right after the backup, and you can check the backup again months later, even when the original is gone:

cd Backup
find . -type f -exec sha256sum {} + > ../Backup.sha256

Later, run this inside the backup folder:

sha256sum -c --quiet ../Backup.sha256

It printed nothing on our intact backup. After we damaged the photo, it printed ./2026/Trip/IMG_002.jpg: FAILED and WARNING: 1 computed checksum did NOT match. A file that has disappeared shows up as FAILED open or read. It won't report files added to the folder after the list was made, because it only checks files on the list. To compare the list with a folder in both directions, use the script in finding which files changed.

Zip and tar backups

unzip -t only checks the zip against itself. On our good zip it printed No errors detected in compressed data of Photos.zip. With four bytes overwritten, it reported bad CRC for the damaged photo. But when zip couldn't read locked.txt, it printed only zip warning: Permission denied, created the zip without the file, and unzip -t still said No errors detected. Check the number of files with unzip -l, or unzip to a temporary folder and compare it with diff -rq.

tar -df compares an archive with the folder. Run it from the folder that contains Photos:

tar -df Photos.tar

On an unchanged folder, it printed nothing. After we edited notes.txt and deleted .album in the original, it printed Contents differ and Mod time differs for the note and Cannot stat: No such file or directory for the missing file. Like a checksum list, it says nothing about files added to the folder later. Our test file added after the backup didn't appear.

What a USB stick drops

Most USB sticks and SD cards are formatted as FAT32 or exFAT. We copied our folder onto real FAT32 and exFAT disk images with cp -r --preserve=timestamps, which keeps the dates like a file manager does. Both refused two items:

• Plan: day 1?.txt, with Invalid argument, because : and ? aren't allowed in file names. Neither are *, ", <, >, | and \.
• cover.jpg, with Operation not permitted, because these formats can't store symbolic links.

Afterwards, diff -rq listed exactly those two. FAT32 also can't store a single file of 4 GB or more, so large videos and disk images fail. And both formats store dates less precisely: FAT32 kept 17:52:40 for a file saved at 17:52:40.90. A size-and-date comparison then flags every file as modified, while diff -rq doesn't.

A checklist for every backup

1. Watch the copy for errors, and don't close the window until it finishes.
2. Count the files and total size in both folders. If they differ, find out why.
3. Before deleting the original, run diff -rq original backup, or the PowerShell snippet on Windows.
4. Save a checksum list inside or next to the backup.
5. Every few months, run sha256sum -c on the backup to catch a failing drive early.

When this fails

Try it in JSON Workbench

To look through the two folders side by side without a terminal, open Folder Compare, choose the original and the backup, and click Differences. It shows missing and extra files right away and lets you open any file to see what changed. It compares size and date, so use diff -rq for the final check before deleting anything. How to compare two folders online explains how it works.