I have TBs of data over 4 drives that I want to move into a single drive that also contains alot of duplicate data

[sparethe4th]

So, during the pandemic, I got a new hard drive for extra storage and a year later it starts to fail, I did have an external drive which I used to move files over. But then my older hard drive started to fail as well, and I don't have enough storage in a singular drive to catch it not to mention I have stuff installed in that drive. And since I don't have a secondary computer to use at the time and not techsavvy, I now ended up with 4 hard drives with definitely duplicate directories but with some modifications on some of them which now totals to around 6-ish TB of data. I side-stepped this issue as a year later I needed to have a laptop for in-person usage and with better storage management.

Now that Windows 10 is not gonna be supported soon and my desktop is not supported and not really too good to continue gaming on, I am planning to make this old desktop to a homelab/nas. What I have planned right now is getting a 6/8TB hdd and dumping the contents of the hard drives into it then process the duplicates to lessen the storage occupied and I can free up those extra drives to be used properly.

As the title says, how would I go around doing this with FreeFileSync?

[Plerry]

Although you can use FreeFileSync (FFS) for a single copy (not move) action of data from one location to another, FFS has no specific benefit for such action over plain file-management tools (e.g. Windows File Explorer).
FFS is not able to solve your duplicate problem for you.
For that, you probably need a dedicated duplicate finder tool (see e.g. here for Windows)

[xCSxXenon]

It would only be feasible if the data structure is identical on all sources. You could run 'update' or 'custom' syncs one by one from each source to the new single destination. This will allow new and non-existing files to get copied, and give you a chance to review duplicate files that exist in the same file path. But as Plerry said, FFS is not a duplicate finder/solution

[lalaupoa]

alldup

[newb]

I am new on FreeFileSync so please do not flame me - teach me if I have wrong expectations, please.
I think I am expecting what @xCSxXenon wrote above.
@lalaupoa: What the heck is "alldup" - please use full sentences if you can.
Would the following work:
(0) I assume that the same files distributed in multiple copies in multiple locations have the same names, and that FreeFileSync can set switches to pay or attention to features such as names, size, dates, etc., and execute based on your selection. Even if some have different names, after having everything in on one disk, in one location, you can run some duplicate discovery programs.
(1) Setup your New drive;
(2) Use FreeFileSync [FFS] to sync Old disk1 to the New drive; after all is done, disconnect Old disk1;
(3) Use FFS to sync Old disk2 to the New drive; after all is done - disconnect Old disk1;
....
(n) Use FFS to sync Old diskn to the New drive.
If Sync in FFS name means sync - you should have all files in the New drive without duplicates. To check find, and run a duplicate discovery program of some sort.

[therube]

AllDup, a very good duplicate file finder.

If your files are not in a directory based structure, Everything 1.5 Alpha, a file finder in general, but very much more, has any manner of ways to discover & deal with "duplicates".