dual boot UEFI installation

Question

I would like to have instructions on how to install ubuntu 16.04 alongside with win8.1(already installed), not disabling UEFI. My problem is that I want Ubuntu in my caddy SSD and if possible to be able to leave windows unaffected if caddy is taken out. I have read a lot about Grub2 but I am getting more and more confused. I am already familiar with Linux usb live usage but that's all. My laptop is a Toshiba LB 50 158 with 4 Gb RAM a 500 Gb primary HD and a 240Gb SSD in a caddy.The CPU is a Celeron N2840 2,16GHz 64bit.

Answer 1

The tricky thing about your configuration is that, in a default Ubuntu installation, the boot process relies on files that span two partitions:

• Ubuntu's boot loader program (GRUB) is installed to the EFI System Partition (ESP), which in your case is likely to be on your internal hard disk.
• GRUB's configuration files are stored in the Ubuntu /boot/grub directory, which by default will be on the main Ubuntu partition, which you say you want to be on your removable SSD.

Thus, in a default installation, everything will work fine when the SSD is installed in the computer; but when you remove the SSD, GRUB will launch, be unable to detect the SSD, and drop back to a grub> prompt. IMO, this is a bug; in fact, I filed this bug report about the problem.

Several workarounds are possible, but they take a bit of planning and deviation from the default installation procedures:

• You can create a separate /boot partition (I recommend 512-1024 MiB) on your main disk and put the rest of Ubuntu on the removable SSD. This will require a small modification to the internal disk, and when you boot, GRUB will show an "Ubuntu" option even when it's not available because the SSD is unplugged. It will be basically functional, though, and it's pretty close to a default Ubuntu configuration.
• You can skip GRUB entirely in favor of a boot loader that doesn't rely on configuration files off of the ESP. My own rEFInd boot manager works well for this. The easiest way to do this is to install Ubuntu normally, then install rEFInd from a Debian package or PPA; however, you may have to periodically correct what I call boot coups -- GRUB taking over the boot process against your wishes. You can tell Ubuntu to not install GRUB at all by booting the installation medium using the "try before installing" option, launching a Terminal, and typing ubiquity -b. This should bypass the problem with boot coups. Note that rEFInd scans for boot loaders and kernels on every boot, so if you take this approach, rEFInd will not show Ubuntu kernel options when the SSD is removed. (It will still show GRUB as an option if it's installed to the internal disk's ESP, though.)
• Instead of installing GRUB to the ESP on your internal disk, you can install it to a second ESP on the removable disk. Ideally, you can configure your computer to boot to GRUB when the SSD is inserted, but this entry will be skipped and the computer will boot via the Windows boot manager when the SSD is unplugged. One problem with this approach is that many ESPs automatically delete boot entries that aren't valid, so your computer may "forget" the GRUB/Ubuntu boot loader when you unplug the SSD. This difficulty may in turn be very difficult to overcome. Another problem is that the Ubuntu installer currently presents very poor control over where to install an EFI boot loader, so getting it set up this way may require post-installation tinkering or mucking with partition type codes to make the internal ESP look like it's not an ESP during installation, then switching it back after installing.
• Install everything in the default way but then rely on the EFI's built-in boot manager to control which OS to boot. In most cases, this requires hitting Esc, Enter, or a function key to enter the boot manager. (Which key you use varies from one machine to another.) You can then opt to boot Windows or Ubuntu (or perhaps other things, like external media) as you see fit. On some computers, this approach can work pretty well; but others have such abysmal built-in boot managers that this approach will lead quickly to frustration or be completely unusable.

Personally, I'd use rEFInd on the internal disk in your situation -- but as I'm rEFInd's developer, I know it quite well and am biased. Any of the preceding options, or variants of them and perhaps other options I'm forgetting, will work. Each has its unique advantages and disadvantages, as just described, so you should pick the one that sounds best for your specific needs and skills.