Hi.
I have created a multiboot ISO with arch Linux and added antiX ISO then did raw write to usb disk. When I boot from usb then antiX cannot find files because checking for available CD-ROM(dev/sr0, /dev/cdrom ). When I start same ISO in VirtualBox, connected as cdrom then it shows no errors.

The question is: can you disable check and look for files on cdrom and use local instead? Is available any kernel options for that?

It sounds like you're trying to create a YUMI pen drive without using any helpful software. Syslinux is a FAT32 bootloader that is much easier to use than Grub2 - about the same as Grub4DOS. Still, a root has to be defined and the kernel directive line needs some parameters. It sounds like these directions are absent and your antiX copy defaults to thinking its on a CD/DVD. Try burning the ISO to a CD/DVD, booting from that, and frugal installing to the USB. I believe this will properly configure your dual boot. You might also be able to easily establish persistence, which is a pain using YUMI. If you can handle Arch, you can figure a way here.

Thank you! I have solved it partly.
Syslinux loads ISO in memory before starting it and it was the problem but I have copied all needed files to usb and added correct entries to syslinux. Everything is fine now but I noticed tha many Linux distributions cannot start from memory because they looking for filesystem file on physical storages.