Mac Pro only loads a black screen
So recently, I purchased an old, 2007, Mac Pro from a friend. It was missing the RAM, HDD, and Video Card. I was able to install 4GBs DDR2 ECC RAM, and a fairly new video card which probably isn't supported but still loads graphics. The HDD, I just used a fairly new WD and a SSD. After trying those drives, and using an USB drive to install, it wasn't working. I tried putting in an old OSX 9 HDD I have from my old Mac Book Pro, wouldn't load. Just a black screen. The only HDD that loads is some 2003 version of Ubuntu off an old drive. I can't figure out how to get it into the install for OS X with the USB drive or to read the old files of that old HDD. Just wants sit there and do nothing, haha. Suggestions? I am able to take a video if needed. (Thanks in advanced, I'm new đ )
Mac Pro, Mac OS X (10.7.5)