When using genkernel-next-64 for booting the identical system from
a) external USB 3.0 mechanical harddisk
b) external USB 3.0 SSD (tested with Samsung T3 SSD)
only a) works.
After countless reboots I found the reason: genkernel-next does not include
the kernel module "uas" in the initramfs by default, only usb-storage.
Missing that the SSD is listed on the USB bus (lsusb) but no /dev/sdX
is created for it, consequently he does not find the root partition
Inclusion of "uas" module in the initramfs makes boot from a USB SSD work.