I copied a backup of my entire VPS (Debian on OpenVZ) on a USB drive, and want to boot from this USB to test something on my server (in fact its duplicate). However, there is no nothing in the boot directory (I've no idea how OpenVZ boots the OS). How can I make the USB bootable?
Do I need to install GRUB on the USB? If yes, how can I do this NOT to affect the GRUB on my HDD (my main computer). I do not want to make a dual boot, it is just temporary booting from USB.
You will need to install a kernel and GRUB. OpenVZ uses kernel virtualization, so it doesn't have a bootloader in the traditional way.
To convert your VPS, you best start a live disk, chroot into your VPS and then install an appropriate kernel as well as grub. (don't forget to generate your boot folder after installing grub)