recreated iso from files of extracted bootable iso is not bootable

I have a bootable iso file of a disk imaging software. I have tested this and it boots without a problem from my multiboot ventoy flash drive both in UEFI and MBR modes. I checked that it shows as bootable on imgburn too. I extracted the contents to a folder on my laptop and software to recompile the iso file. To my amazement the recompiled iso is NOT bootable and shows such in img burn...Iso file size is marginally different too! The two isos are IDENTICAL in terms of file structure and individual file/folder sizes.

Why would this happen?

I would like to understand what is going wrong here and why

Thank you

how did you create the iso ? You need to use efisys and/or etfsboot for the iso boot sector

you could use cdimage or oscdimg or mkisofs to create the el torito


like this:

REN "ISO\BOOT" BOOT
REN "ISO\BOOT\ETFSBOOT.COM" ETFSBOOT.COM
IF EXIST "ISO\efi\microsoft\boot\efisys.bin" (
COPY/Y "ISO\efi\microsoft\boot\efisys.bin" "ISO\BOOT\EFISYS.BIN"
)

IF EXIST "BIN\oscdimg.exe" (
"BIN\oscdimg.exe" -bootdata:2#p0,e,b"%TP%\ISO\BOOT\etfsboot.com"#pEF,e,b"%TP%\ISO\BOOT\Efisys.bin" -u1 -udfver102 "%TP%\ISO" "%TP%\7PEx64%wimlang%.iso"
) ELSE (
"BIN\mkisofs.exe" -iso-level 4 -udf -r -force-uppercase -duplicates-once -volid "SIW2" -hide boot.catalog -hide-udf boot.catalog -b BOOT/ETFSBOOT.COM -no-emul-boot -boot-load-size 8 -eltorito-platform efi -no-emul-boot -b BOOT/EFISYS.BIN -o "%TP%\7PEx64%wimlang%.iso" "%TP%\ISO"
)

An easier way for beginners is to extract the boot image from a known working iso and insert it into the new one.

I use winiso for convenience , as far as I know it is the only isohandler that can extract and insert multiboot images ( support both efi and bios boot ).

WinISO - A fast and convenient ISO image file editor

WinISO : Multi-Boot Image File (.bifx) Format

you only need either oscdimg or mkisofs . efisys and etsfsboot are already included in the windows distribution.

mkisofs.zip

easier for beginners to use winiso and avoid commands.