What is the function of the WS.DAT file in the SOURCES folder of ISO?

I want to add EI.CFG to Windows 10 ISO and then add that ISO to a multi-boot USB flash drive.
So I extracted the content of the ISO to a folder, added EI.CFG to SOURCES and then used IMGBURN to create an ISO from the folder.
I then compared the original ISO with the new ISO and found that the WS.DAT file was missing.
Tried the same exercise without adding EI.CFG, and again the WS.DAT file was lost.
Tried the same with Win11 ISO; same result.
Can't find any reason why IMGBURN would skip the file.

Thanks in advance for info.

I wish that I had kept better track of this issue when I was doing the very thing that you are doing now. Back when I first started injecting updates into Windows images, I was doing some tests and noticed that images that I was creating were off ever so slightly in size even though they should have had 100% identical contents. I vaguely recall that the difference came down to what I think was time and date stamps somewhere, but for the life of me I cannot recall if it was in a WS.DAT file with any certainty.

However, I have a couple of questions for you:

1) In what images are you seeing a WS.DAT file? When I download images from Microsoft, I see no WS.DAT file in the original ISO images. Are you sure that the problem is not that this file is missing, but rather that the problem is the presence of this file? Is it possible that DISM (or something else) is adding this file? I seem to vaguely recall something was added accounting for the size difference in my images, but I apologize - I don't know if it was a file called WS.DAT.

2) Does Windows install fine from your images WITHOUT the presence of this WS.DAT file? If it was something needed for the installation of Windows, you can be sure it would generate an error if it was needed and then was missing. As I noted, I don't have a WS.DAT file but my Windows images all work flawlessly.

I will try to do some experimentation in the morning (it's about 11:30 PM for me now). As I noted, the original Windows ISO images downloaded straight from Microsoft have no WS.DAT file, so I bet something is adding it.

I'll report back what I find (if anything) tomorrow after I have had an opportunity to do some testing.

I thought I would do a quick search before I hit the sack, and I found the below reference to that file. I suspect that it has something to do with the Media Creation tool, a sort of flag to indicate that the media was created using Web Setup. Try downloading the ISO directly and I suspect that you will find, as I did, that there is no ws.dat file.

See Full Details about a Windows 10 ISO file or USB

Search for ws.dat on this page. This seems to confirm that it has something to do with the media creation tool:

windows 10 - Safe way to verify that a Microsoft ISO has not been tampered with - Super User

hsehestedt said:

Search for ws.dat on this page. This seems to confirm that it has something to do with the media creation tool:

windows 10 - Safe way to verify that a Microsoft ISO has not been tampered with - Super User

It is indeed from the media creation tool.
Content is:-
[WebSetup]
ClientVersion=10.0.19041.572
IsUpgradeNow=FALSE
InstanceId=9dc94478-9757-4a20-a75e-33f797c7268b

So I think I can do without it.

Great! Okay, time for some sleep for real now