I have a Mac Mini mid 2010 running Yosemite. When I power it on, the gray apple logo screen appears and the progress bar moves a third of the way then creeps to around 50% after 40 to 60 minutes. I tried to power on and use CMD R which did not take me anywhere other than the same screen. Tried holding Option and chose Recovery 10.10.3 drive. Same result. Tried choosing my external backup drive where Time Machine backups are located. It showed the apple screen then a Kernal Panic error displayed and rebooted. I'm at a loss of what to try and do. I attached a pic of the kernel error.