I have a mac pro desktop that has recently stopped working. I can turn it on, the white apple logo will appear, it will begin loading the OS, but when the loading is complete and I should see the login screen, the screen just goes black. I have tried everything; - swap out memory with known working sticks - PRAM reset - SMC reset - Graphics card swap - Pulled all hard drives - tried loading boot disk (load then blank screen) - etc etc If there is no boot drive, the usual question mark folder will appear, I can boot in verbose, but no errors or warnings appear, i can boot into single user mode, and browse directories, but no OS will load! Anyone seen this before? and can possibly help?