Today, my PC did not boot as usual to Debian. BIOS could not find my /dev/sda and was looking for the netboot image. I restarted my PC and got into the BIOS boot setting menu. Hmmm.... my first SDD (/dev/sda) was missing. My second HDD (/dev/sdb) was there. But I did not put the Grub boot-loader there. No wonder it did not boot.
I have a 32GB USB3 stick with the full Debian system. It is not a live CD image USB stick but a HDD formatted and encrypted system. Though it is not the fastest system, it is very light and usable. I plugged it in and powered it up. It booted OK but /dev/sda was still missing. While it booted, I saw "ata1: COMRESET failed (errorno=-16)" . So this ata1 SSD cannot be accessed from BIOS nor Linux. Sigh ...
Looking around the web under the USB stick system, I saw some people were talking that the loose serial ATA cable sometimes causes this message. Since my PC is a laptop, It has no flexible cable but has an on-board connector inside for the SSD.
Hoping my problem is just a bad connection problem, I crack opened the back panel of my PC. The SSD looked fine. I unplugged it from the connector and reinserted back into the connector. After repeating this several times to be sure, I closed the back panel and booted.
It boots as expected into Debian. Looks like everything is fine.
SMART Error Log Version: 1
No Errors Logged
If you have any boot problem like mine, please reinsert your SSD to the connector like I did before you panic.
PS: This Crucial/Micron RealSSD m4/C400/P400 M4-CT256M4SSD2 previously had a problem. A firmware bug made it read-only. The firmware updates fixed my Debian system on this SSD. I could fix this without Win*** OS since the firmware update was on a bootable disk image file.