The \_ISO\CONTIG.ISO file (if present) is only used by the E2B Menu system when it finds a payload file that is not contiguous.
Easy2Boot has a feature that means that you don't have to make all your Linux ISO files contiguous (though it will be much faster and more reliable if you do!).
If a payload ISO file is not contiguous, E2B can copy the contents of the whole ISO to the \_ISO\CONTIG.ISO file which is a contiguous file. E2B will then boot from CONTIG.ISO instead of the original, non-contiguous ISO file. However, this feature is only useful for .ISO files that are reasonably small as it takes time for E2B to make a contiguous copy of the ISO (and it does this each and every time you run the same ISO). This feature is mainly for Linux users who may have difficulty in defragging the E2B payload files, due to the lack of good Linux file defrag/contig utilities. Note: If you made your E2B drive using Windows and the Make_E2B.exe GUI, the E2B drive may not contain a CONTIG.ISO file.
Note: Ventoy and some options in the agFM menu system do not require the ISO files to be contiguous, so they may be more successful than E2B if the file is not contiguous.
Tip: When running from the E2B drive on some systems, the E2B copy process can be quite slow - the copy time can often be greatly improved by using the 'Install Grub4DOS USB driver' menu option first (in the Utilities Menu in later versions), before you select the ISO file. You can also hold down the SHIFT key as E2B starts to boot to load the grub4dos fast driver. The CONTIG.ISO file must be at least as large as the ISO file you are trying to boot from (max 4GB) . The E2B download contains a 500MB \_ISO\CONTIG.ISO file which you can copy to the USB drive if it is not present. If you see this message:
WARNING: /_ISO/CONTIG.ISO is smaller than ()/_ISO/xxx/yyy.iso - cannot copy!
then you will need to increase the size of the CONTIG.ISO file or make the source ISO file contiguous (e.g. using WinContig or Defraggler). It does not matter what is in the CONTIG.ISO file, as long as it is contiguous and is the same size or larger than the non-contiguous ISO that you are trying to boot from.
How to make a CONTIG.ISO file
1. Run "\_ISO\docs\E2B Utilities\Make_CONTIG.ISO\Make_CONTIG.ISO.cmd" from the E2B drive to quickly make a new CONTIG.ISO file
Copy a large file - e.g. 3.8GB, to the \_ISO folder of your E2B USB drive and rename it as CONTIG.ISO. Any file will work as long as it is larger than the largest ISO that you might use (up to 3.9GB). You can also create a large file using the RMPrepUSB - Create Ext2 FS button if you wish.
2. Then run WinContig on the E2B USB drive to make sure that the CONTIG.ISO file is contiguous (or use DeFraggler or WinContig just on the \_ISO\CONTIG.ISO file).Now when you add any new Linux ISO files, if you forget to make them contiguous, E2B will copy the ISO file to CONTIG.ISO and boot from that file. For more details see my blog post here.
Note: Do not exceed 3.99GB for the CONTIG.ISO file (even on an NTFS E2B drive) as the size check in E2B will not fail due to a grub4dos bug. If you are short of space on the USB drive, you can just delete the CONTIG.ISO file at any time (but you then cannot use this feature).
Tip: To boot to grub4dos 0.4.6a and load the USB 2.0 grub4dos driver, hold the SHIFT key down as E2B starts to boot to load the USB driver.