I'm using putty to connect to the APU over a serial port. First time it booted, I was able to make inputs and to select the boot device. Because I'm trying to boot from USB I selected usb1 and pressed E for Exit and Save.
It didn't booted from the USB so I took the power away. After that it always starting the "SageBIOS Memtest86+ 5.01" and I can't make any input in putty.
How can I stop this Memtest and how long do I have to wait until it booted a Tiny Core Linux?
I think, it's because I already inserted the empty SSD. That's what I found:
"If you're booting from an SD card or USB stick as soon as the SSD is detected the boot loader will switch to it and disable the USB/SD boot and the system will hang"
But how can I enable USB Boot again?
So it turned out that I disabled the serial console, it saved it with S1.