[NAME] Area-51 ALX 2006  just want to give it an update. My issue got fixed after I change my SSD for a new one. But my old SSD also works fine without any issues. If only I use my new SSD as my boot drive and the old one as storage.  If I boot from the old one my pc crashes and SSD disappears from everywhere. So I can confirm that its an firmware issue from Toshiba KXG5 series SSDs. Maybe after the update it can be used as a boot drive like before. But I might keep one using the new one as my boot drive. As the new one has a 5 year warranty period. 
I have had this issue before too as the SSD doesn't get detected when you first boot the computer.  This update : Toshiba Solid State Drive Firmware Update for KXG50ZNV256G/KXG50ZNV512G/KXG50ZNV1T02 | Driver Details | Dell US fixed the issue when I had it before.  Now for the past 11 months I was getting tons of BSOD on High load. I posted it here : Solved: Re: 17 R4, Blue Screen Issue - Page 2 - Dell Community  Then these 2 updates :  1. Intel Management Engine Components Installer | Driver Details | Dell US 2. Alienware 15 R3/17 R4 System BIOS | Driver Details | Dell US  FIXED MY BSOD ISSUES, Thanks Dell. Although you took a lot of time.    Right now after the update my pc works fine on any loads, But the SSD tends to disappear every time I turn my laptop on for the first time of the day. After 2-3 power cycle the SSD operates normally.  As we all know its an firmware issue, So please Dell update the firmware of the SSD. Thank You !! 
17 R4, SSD not detecting, after a few restart it comes back again