I'd like to perform a full install of Ubuntu Desktop, in legacy mode, on a GPT usb disk. The disk in question is a 256GB Sandisk USB flash drive, and I'm using a 120GB partition as my root (/) partition. The disk is a high-performance drive, with write speeds north of 150 MB/s when formatted to ext4 (which I will be using for my root partition).
I've tried doing this, several times, using an installation disk running in legacy mode formatted using MBR; however if the target disk (that I specified above) is formatted using GPT, the installer automatically installs Ubuntu in UEFI mode with no option to specify otherwise. It only installs in legacy mode if the target disk uses MBR.
This poses a challenge, as I'd like to enjoy the benefits of GPT (particularly its support for up to 128 partitions), but while having Ubuntu in Legacy mode.
Often, in response to questions like this, commenters have asked the motivation for opting to use legacy rather than UEFI mode. My reasons for choosing legacy are: compatibility, I'm installing to a USB stick that will be used accross multiple PCs, and I'd like it to be compatible with older machines that only support legacy booting; and simplicity/speed, as having installed Ubuntu in both legacy and UEFI mode on USB drives several times, I've (somewhat surprisingly) observed significantly faster boot times using legacy mode (in my understanding this is due to a simpler boot process involving fewer hardware checks, etc.) than UEFI mode (which can be intolerably slow to boot, even on faster USB drives). I've also had more issues with UEFI installs on USB drives - for example, every time I perform a USB install of Ubuntu using UEFI, I have to manually move EFI files around to the right places before being able to boot normally (otherwise I have to manually select which EFI file to boot from using the file browser in my PC's boot device chooser) - I never have had to do this using legacy mode.
I'd be very grateful if someone could tell me how I can install Ubuntu, in legacy mode, on a GPT USB disk.
