Reading related posts it seems I have a blend of other's issues. But the final blow was plugging in a nextbook tablet to download some pdf files, and THEN I booted the old Mac Pro 2008 with Yosemite 10.10.3 or 4, can't recall exactly now since I can't check it. I 'assume' having the tablet plugged in on boot confused the boot up sequence?
I've tried every startup key configuration in the official Apple list, except trying the few 'over the internet' options i.e. 'Option-D: Apple Hardware Test/Apple Diagnostics over the internet,' 'Command-Option-B Start up from OS X Recovery over the internet,' because my hardwired modem is on the computer I'm using to get help. I'll power this down and try it now, but if that doesn't work, which isn't likely, it's still in White Screen Hang. It was also black screen crashing before this and restarting over and over, intermittently... if that may be contributing to this issue. Thanks.