rEFInd

I installed Ubuntu Linux on my dyanbook N40, but when I tried to boot it, it displayed "System Reset" and kept rebooting. I had no choice but to try various other distributions. At last, I noticed that the boot options of the Slackware installation USB allowed me to select a boot file other than USB. It can select not only other Linux distribution but also maybe Windows, I guess.

I see... I didn't know there was a way to do that (:-). So, I have established the operation of booting Ubuntu on my Dynabook N40 from Slackware's installation USB by pressing F12 (:-) every time I start up my computer.
I had been using Ubuntu with this operation for a while, but I decided to install rEFInd, thinking that there must be a smarter way nowadays. I found "Install to Storage" option on the rEFInd USB boot screen, so I tried it and ,after all,  Ubuntu booted from Dynabook N40 without F12. The combination of rEFind + Linux is the best for Linux installers who are suffering from UEFI problems. I wish they should make it a standard feature (:-).

UEFI

 

 I installed several Linux distributions on my dynabook N40, and among them, Ubuntu install USB can work as Live Linux. So I can try out Linux as is. Ubuntu is very convenient because it recognizes Wifi as default for dynabook N40, whereas other distributions often Wifi does not work at first stage. That is  ,of course , Ubuntu is the leader of Linux for clients. It is a long time ago when we had to spend a whole night compiling kernels and modules with different options to install devices that were not recognized by default (:-).

So the installation of Ubuntu on the dyanbook N40 ended easily, but when I disconnect the USB and rebooted, to my surprise, it did not boot properly (;_;). The "System Reset" message appeared for a moment on the startup screen, and then the system just kept rebooting.
I have investigated various things, and it seems to be related to the UEFI. The transition from the old BIOS to UEFI is progressing, but the dynabook N40 is a product from the early days of UEFI and does not seem to be very sophisticated. I tried several countermeasures such as tweaking GRUB2, using repair tools, and of course, changing UEFI options, which I could find on the net, but none of them worked.
I had no choice but to try various distributions other than Ubuntu, such as Slackware and Arch Linux, but that didn't work either. CloudReady, which is supposed to be based on Linux, was able to boot from the dynabook storage without any problems, so there must be a solution to this problem.