I'm trying to install Elementary OS on a USB drive so I can carry my OS with me. I have installed the ISO to the pen drive and can boot into the desktop no problem. When I come to do an install the installer doesn't see the pen drive as a disk it only sees my internal HDD's. Any ideas? Thank you
You can use a tool like Fedora LiveUSB creator. You can simply download your choice of Linux Image and generate on the go pen drive. I have successfully been using it with Ubuntu 14. https://fedorahosted.org/liveusb-creator/
The safest way is, I think, to use a virtual machine such as VirtualBox; mount the iso of the install system, and add the usb drive to the VM.
Otherwise, many installers will switch the boot loader portion of Grub to point to the USB drive. Then, the HD system will not boot unless the USB drive is present. I had this happen, but the USB drive got destroyed or became defective, and I had to do a bit of hoop-jumping to fix it.
Another way is to first disable the HD in the BIOS, then install the system to the only possibility, the USB drive.