Can I unplug USB media installation while installing?

I heard that while installing via USB (or Live CD), all files are loaded into ramdisk? And as far as I know, ramdisk is just RAM that behave as HDD.

My RAM is 4GB and ubuntu installation media is just (more or less) 1 GB. So basically, if all the files are loaded to RAM, RAM still have 3 GB to be used as actual RAM. So, is it save (or possible) to unplug Installation media while installing?

Not all of the information is loaded into RAM. If you pull the drive Ubuntu will stop working because the commands are still on the USB drive.

I just tested this out, and I am replying to this without my USB drive in the system and it is working fine while I am still in the Try Ubuntu.

On your USB drive, modify boot/grub/grub.cfg and add toram to the following so both the Try and Install will work without the USB drive in:

menuentry "Try Ubuntu without installing" {
    set gfxpayload=keep
    linux   /casper/vmlinuz.efi  file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash toram --
    initrd  /casper/initrd.lz

}
menuentry "Install Ubuntu" {
    set gfxpayload=keep
    linux   /casper/vmlinuz.efi  file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash toram --
    initrd  /casper/initrd.lz
}