Before you retire a drive
Short answer: you cannot work this out by looking. Folder names, file dates and directory structures diverged the moment you started copying between the drives. The only question that survives all of that is whether the contents match — and it has to be asked across both drives at once.
Scanning both drives and exporting the full report is free, with no account. A licence is only needed to move anything.
There is also an order of operations that makes this safe, and it isn't the obvious one. It's further down, under the safe order. Read that part even if you use something other than our software.
On a single drive, a duplicate is just wasted space. Across two drives, duplicate and backup are the same word describing the same file, and nothing in the file itself tells you which one you meant.
The history is nearly always the same. One drive filled up. You bought a second, larger one and copied some of the first across — probably the folders you were working on, probably not all of them. You kept shooting onto the new drive. Months later you copied a couple of old jobs back the other way because a client resurfaced. Somewhere in there a backup tool ran once and you can't remember what it was pointed at.
What you have now is three categories of file, thoroughly mixed together:
That third category is the reason this is dangerous. It is usually small, it is usually old, and it is the material you would never think to check for — a job from four years ago, a personal folder, the one shoot that never got copied forward because the drive disconnected halfway through. Deleting from the old drive what you believe is on the new one is how people lose the only copy of something.
Folder names diverge immediately. 2019_Smith_Wedding on one drive is Smith Wedding (June) on the other, because you reorganised at some point and only reorganised one of them. The photographs inside are identical. Sorted side by side, they don't line up at all — and two folders that do line up by name may hold different selects from the same shoot.
Copying rewrites timestamps, and which timestamp it rewrites depends on how the copy was made. Dragging in File Explorer, robocopy with and without /COPY:DAT, and backup software all leave different date fingerprints on identical content. A file whose modified date is three years newer than its twin can still be the same photograph, byte for byte.
Size is an aggregate, so at best it tells you that something differs somewhere. It cannot name the file. And it produces false alarms of its own: Windows reports both Size and Size on disk, and the second depends on the cluster size of the filesystem. A drive formatted exFAT and a drive formatted NTFS will legitimately report different totals for identical content.
There's no facility in File Explorer for comparing two volumes. Nothing built into Windows answers the question "which files on E: also exist somewhere on F:" across a whole drive.
This is the trap worth naming, because plenty of tools are built this way. They take a folder tree, find groups of identical files inside it, and offer to delete the extras. Point one at both drives at once and it may well be right about what's identical — but it has no concept of which drive you intend to keep, so its "keep one, delete the rest" logic can leave your only surviving copy on the drive you're about to wipe. Being correct about duplication is not the same as being correct about which copy survives.
Only one comparison is trustworthy across two drives: reading both files and checking whether their contents match. The difficulty is that doing that naively means reading every byte of two archives that might total twenty terabytes, which is a scan measured in days.
DuplicateFinder Pro runs three tests in order, cheapest first, and a file only reaches the next test if it survived the one before.
One deliberate exception, which the app labels rather than hides: RAW files sharing a filename but not byte-identical are still grouped so you can see them side by side. Those rows read stem+size in the Match column instead of md5, and if the sizes differ by more than 5% they are marked Review rather than flagged. Every row tells you which test found it.
This is the part most guides skip, and it matters more than which software you use.
The instinct is to compare the two drives in place, then delete the duplicates from the old one and wipe it. That works right up until the moment it doesn't, because it leaves the files that exist only on the old drive completely unexamined. A duplicate finder finds duplicates. Files with no counterpart aren't duplicates, so they never appear in the report — they are invisible precisely because they are the ones at risk.
Copy everything from the old drive onto the new one. Don't merge folders, don't tidy — drop it in as a single dated folder and let it be untidy. Now every file you own is on one drive, including the ones that were only ever in one place. Then run the scan on that drive alone, and reduce the doubled files to one copy each. Nothing unique is ever exposed, because nothing unique was ever the only copy during the operation.
Run the two-drive comparison, quarantine the redundant copies on the old drive, and let that free the space. Then copy what remains on the old drive — which is now, by definition, the material that exists nowhere else — onto the new one. Only after that has landed and been verified does the old drive get formatted.
In both routes the same rule holds: the drive you plan to retire is never the drive you delete from and then trust.
Scanning and the full CSV and HTML reports work without a licence, so you can run all of this and see the result before deciding anything.
Most duplicate finders are written for one disk. These are the behaviours that only matter once a second drive is involved, and they're the reason this tool exists at all — it was written to consolidate a 25 TB archive spread across external drives.
DSC_0001.NEF exists on all of them and means something different each time. Identical basenames from different drives are given unique names in quarantine, and each keeps its own recorded origin, so a multi-drive cleanup can be undone one file at a time without ambiguity.
If a file's original location is on a drive that isn't currently connected, the app names that location, leaves the file safely in quarantine, and offers to continue with the rest. It will not redirect anything to a plausible-looking substitute path.
On cross-folder and cross-drive matches, each flagged file is hashed against the copy you're keeping right as you click quarantine — not only when the scan ran, which on a multi-drive job may have been hours earlier. If a file changed in between, it is skipped and named.
Checkpointing at phase boundaries means a paused or interrupted multi-hour scan resumes: everything already finished stays finished, and only the phase in progress runs again. Over two drives, a scan that has to restart from zero is a scan that never completes.
Four things, in order. None of them takes long, and the drive isn't going anywhere.
Formatting is the one step here with no undo. Everything before it is reversible, and there's no prize for reaching the end quickly.
Yes. The comparison reads files from both drives during the scan, so both need to be mounted and stay mounted. This is also why a drive that sleeps on idle is worth plugging into a powered hub first.
No. Content matching reads the files themselves, so cluster size, filesystem and capacity are irrelevant. A file copied from an exFAT drive to an NTFS one is the same file, and is recognised as such.
The one in the first folder listed in the Folder Duplicates tab. The app states this rule on screen above the scan button, and every result row carries a Role column naming whether it is the copy being kept or a flagged copy. To change which copy wins, remove a folder from the list and add it back in the order you want. The app never tries to guess which drive is your archive.
They aren't duplicates, so they will not appear in the report — which is exactly why they are the risk. Copy the old drive's contents onto the keeper before deduplicating, or copy what remains after quarantine, but do not treat an empty report as evidence that a drive is safe to wipe.
Yes. The staged tests mean most files are eliminated on size alone without being read, and scans are checkpointed so an interruption doesn't cost you the whole run. Scanning one file category at a time keeps individual passes shorter.
No. Flagged files are moved to a quarantine folder on the same drive and recorded, with one-click restore to their exact original location. Emptying quarantine is a separate decision you make yourself, and restore and permanent delete are never locked behind the licence.
No. Scanning, the Storage Map and the full CSV and HTML reports are free with no account. The $34 licence is only needed to move flagged files into quarantine.
The free scan tells you how much of the old drive is genuinely redundant before you commit to anything. If the answer turns out to be "less than I thought", that is a useful thing to learn while both drives still exist.