Having trouble using crack on Doom 3.
 

It always says "Files on this CD-rom drive are read only."

Do you own the original? Sounds like you're tryin to copy the crack to a mounted ISO of the CD or directly onto the CD in your CD drive! Either way won't work as the message says, CD's are read only (meaning Pressed CD's and finalized CD's).

Install the game then copy the crack to the game directory on your hard drive.

Thanks