MultiROM is a tool which allows you to install multiple ROMs to your Galaxy S4. It allows you to install Android ROMs or other operating system not only from SD card but also from a USB drive. If you want to learn how to use MultiROM and install multiple ROMs to Galaxy S4 visit this article.
During installation MultiROM allows you to select ROM location. To install a ROM from a USB drive, plug the USB drive and wait a while until it recognize the device, press refresh and you will see the USB partition. Select the location and proceed with installation. NTFS and FAT32 partitions are supported.
If you want to use other than the FAT32 default partition, format it on a PC. If you are installing to NTFS or FAT32 partition, recovery will ask you to set image size for all the partitions. You can’t change this anymore so choose carefully. FAT32 has a maximum of 4095 MB per image, this is system limit so you can’t do anything about that.
Installation to USB drive takes longer because the flash drive usually is slower and it needs to create the images. Enumerating USB drive can take a while in MultiROM menu so when you press the USB button in MultiROm you have to wait a while until it search the USB drive. It search it by itself so you don’t have to do anything.