I've been reading about how to upgrade to a bigger hard drive and it seems nearly impossible to due because of my missing components at the house. I have 2 computers, 1 doesn't use IDE and the other does, problem is that the computer that is older has IDE cables but has no CD drive, so if I wanted to create an ISO with XboxHDM, I wouldn't be able to burn it to a disk and boot off of it.
Is it possible to boot XboxHDM from a flash drive, maybe mount the ISO, or somehow run it another way?

Use xboxhdm 2.2, which boots from USB. You can find links to that and a list of the other available softmod HD upgrade methods by following the link in my signature.

Just use chimp 2.6 to clone the drive ... much easier ....