As in the subject, if my external keyboard is attached to my Dell Vostro 3750 laptop (via usb 3) - Ubuntu does not want to boot - I see only black screen. Could you please help me?
If you've installed Ubuntu from a USB, you might have asked your laptop to boot from one of your USB ports when you installed it. And because of that each time you try booting and there is a USB device plugged in such as your keyboard (some peripherals have software incorporated), it tries booting from there, it might only happen on that USB port because you might have had the choice of a specific port (if this isn't the case ignore this post).
But if it is, check the boot order of your laptop in the Bios Boot Menu, and make sure your laptop is set to Boot from your hard-drive first then CD then USB. That way it won't try to boot from something else and it should work properly with your keyboard plugged in.
I also have Dell Vostro 3750 i7 quad core.
If there is anything plugged in in USB3 port(s) - Ubuntu would not boot properly and would just show blank screen or a cursor blinking in the top left corner of the screen.
The solution is to unplug whatever is plugged in USB3 port(s) - say mouse or USB stick or 4 port hub or whatever, and Ubuntu would boot up normally. It boots normally if peripherals are plugged in USB2 port.
Worth mentioning is that these problems do not exist when booting Windows 7 (64 bit, preinstalled on this Dell Vostro 3750): this USB3 problem happens only with Ubuntu.
I faced this problem when installing Ubuntu too. I had the USB plugged into USB3, but when I plugged it into a USB2 port, Ubuntu 12.04 LTS installed in a second.
It seems USB3 and Ubuntu boot or install process have some problems/bugs.