If you REALLY want to use the AFNI GUI to export raw data slices, you have to do the following:

* In the image viewer, click on Disp
* In the Disp controller, click all the "Save to" buttons off
* Also click "Save One" off (unless you just want 1 slice)
* Now the "Save" button in the image viewer should read "Save:bkg" which means it will save the background image data.
* You can now use "Save:bkg" to write out the raw data from the Underlay dataset.

The format (usually) is a small ASCII header indicating the data type and image dimensions, and then the raw data.
The header (if present) is of the form

MRI d x y\n

where
* d = data type: 0=byte 1=short 2=int 3=float 4=double 5=complex 6=rgb
* x = number of pixels in each row
* y = number of rows
* \n means the ASCII newline character

Note that int and double are not supported AFNI dataset types, and so data type codes 2 and 4 will never be written by the image viewer!

However, the header described above will not be written if the data type is short or byte and the image dimensions are square and are both powers of 2 from 64..1024 -- then the file will be just the raw data and nothing else. This "feature" is a holdover from the early days of AFNI (1994-6) at the Medical College of Wisconsin, since these were the data formats we received from the EPI reconstruction software and there were a bunch of programs that expected "really raw" image data like this.

I don't believe this "save the raw slices" feature has been used in AFNI by anyone for many years. I have not explained it for well over 10 years, and you are basically lucky that I have an excellent memory.