I apologize in advance if this has been posted elsewhere, I have searched but have found no solutions. My system profile is at the end of the post. I purchased my Mac Pro 5,1 last month as a used video editing machine. Soon after I started using it, I could not get it to a Mavericks desktop without holding down the option key and manually selecting the boot drive. I do not recall if this started with the Samsung 1TB SSD that came with the machine or when I replaced it with an Accelsior e2 PCI SSD as my boot drive or even the upgrade to Mavericks from Mountain Lion. Now from a cold start, I get a white screen, Apple Logo and spinning gray dots. Normally nothing happens, I shut it off, restart it, hold option key and before I even selected a drive I get the "abnormal shutdown, do you want to reopen all the windows" dialog box. If I cold start it and hold down the option key it shows up as a boot drive, as does Windows 7 (separate drive) and my External Drive with its recovery partition plus the recovery partition on the Acccelsior e2. From here it works fine. I can restart the machine, boot to Windows, etc without issue. As soon as I reboot it from a cold start, back to no boot drive. Windows 7 works as expected. Things I have tried: Startup Disk in System Prefs is set as my Accelsior e2. Strangely there is no lock icon present. My Windows boot disk doesn't show up either. I was using Paragon NTFS for Mac which I uninstalled and unplugged the drive. No change. I booted a to a recovery partition, used terminal diskutil, fdisk, flag, to set active partition. No change. I reinstalled Mavericks to the boot drive from a recovery partition on my external drive. No change. Turned off external drives. No change. Installed BootRunner. No change. I spoke with OWC and the vendor I bought the computer with. No change. Other Strange Behaviors: Apricorn Velocity x2 card hangs the system at boot even with no drive installed and in any PCI slot. No lock icon on Startup Disk. Tonight from a cold boot I got a white screen, no apple logo, a couple of hard resets later the screen displayed a bunch of text I could read fast enough (think blue screen of death memory dump in Windows) and then said "you have encountered an error, restart". From here I could select option and pick my boot drive. System Profile: 2010 Mac Pro 5,1 12 x 3.46Ghz proc 128GB RAM nVidia GTX 770 4GB vid card with custom firmware to provide boot screen 4x4TB RAID Accelsior e2 1TB PCI SSD in PCI slot 2 (x16) Startech eSATA card in slot 3 Blu-ray burner in optical drive bay 1 Samsung 256GB EVO 840 SSD in optical bay 2 (no unplugged) with Windows 7 Samsung 1TB EVO 840 SSD in eSATA enclosure connected to Startech card Newer Tech Mini Max Blu-ray burner, SD reader and 4TB drive for backup connected to Accelsior eSATA port Phillips 27" 2560x1600 monitor connected via displayport Apple bluetooth keyboard, Logitech USB mouse USB scanner, printer, TV tuner Not using Boot Camp drivers as they lock up Windows 7 with my RAID.