Copying Data from Multiple Protected Workbooks into Master

This is the way I am interpreting this requirement.

You open a workbook, get a piece of data and put in in a specific cell. Then you open the next workbook, get the same piece of data and put it in the cell underneath the one you just got.

I have several questions: Are the books in question in the same directory and are they the only books in the directory - or do you know the name of each book?
Will each book be set up so that I always go to Cell A4 (for example) and always put that in Cell C1 and for the next book, put the same piece of information in Cell C2. etc.?

You probably won't have to protect, unprotect the workbook if you are only reading data from it. Simply close the workbook without saving and it will stay protected.

We'll need to know a couple more things:
1. The directory structure
2. A couple of samples of what the source workbooks look like.
3. A sample, mocked-up workbook of what the desired result would look like based on the sample input.

VBA can go to a directory and process all the files of whatever type you specify. We don't need to know the names of the files. However, we do have to know the names of the directories. Also, I recommend that for each directory, you have a sub-directory named Processed. After a file is processed it will be moved there so it does not get processed the next time the program is run.

I have a couple more questions: Will Machine, Serial, and Date always be in cells A1, C3 and G4, respectively - I will press on as if they are.

Is there a password on the workbooks?

I think this does what you want it to do. It does not matter if the workbooks are protected. All the program does is read data. It does not try to change the workbook nor does it even try to save it.

I can't duplicate the issue. It copies the cells OK. This might be one of those wonderful compatibility issues between versions of Excel.

I'm using Excel 2016 - when I click on Protect Workbook, and enter and confirm the password, the icon still reads "Protect Workbook." There is no indication that it is already protected. I was expecting the icon to change to "Unprotect Workbook."

But when I click on "Protect Workbook" I get the prompt to enter the password to "Unprotect Workbook"

The only suggestion I can make is to double check that you have the files in the pickup directory and that the proper cells are populated. Attached is what I used as my "fodder" for testing.