I have a external hard disk 1 TB of storage and I already have Windows on it -- 350 mb fat32 and 100 gb ntfs with 800 gb about unused space. Now, is it possible to install RedHat on it? I don't want to remove the Windows partition.
You will need to put the installation media somewhere bootable, either on an install DVD if you have a DVD reader, or on a USB drive, or on a network server, or on your external hard drive.
You'll then need to boot the installation media from a boot manager. When I last installed Linux on a Mac I used gummiboot, now named systemd-boot as the boot manager.
The essential pieces to have available on storage or via the network are grub*.efi, grub.cfg, the kernel, vmlinuz, and imitramfs, initrd.
The "stage2" SquashFS ("LiveOS"?) can be fetched via the network with the right inst.stage2 or inst.repo parameter setting.
When you run the installer you can choose to install to the unused space on your external hard drive.