To preface this; A user makes room on their system to install a copy of Ubuntu MATE. Said user does not want to worry about conflicting bootloaders, so the user plugs a 512MB USB key to install the bootloader on.
Is it a good idea these days to do it?
Hi @tiox,
it depends entirely on what you want to do I suppose, read through the following link which has various scenarios:
Potentially, it may be a fine solution. I’d say this should be a last resort.
A new can of problems could arise, like another media being plugged in that affects the boot order; boot problems if cannot find /boot (or Windows 
), or if kernel updates do not regenerate entries for GRUB.
That’s the intent of using a bootloader on a USB drive; if you have a whole whack of operating systems on the same disk, that’s fine, just, Windows’ MBR should remain unaffected… right?
Else, I don’t know what else I can say about this particular topic. There are tools like EaseUS Partition Master which makes partition editing as easy as gParted’s on Windows, and if the GRUB order gets cocked up, who cares, just boot into a system’s live session from another media and try to fix it there… right?
It should, indeed. That's basically what I did before switching to Linux as my main OS - installed it including the bootloader for testing on a USB drive - it didn't touch the Windows bootloader and I just used the BIOS bootmenu to select the USB drive to boot my test installation of Linux.
If you have another drive, just install the bootloader there (a thumb counts as another drive).
E.G. Let’s say you have SSD1 and HDD1. On SSD1 is the Windows bootloader, then you put grub on HDD1. There won’t be conflicts. When you want to boot thru grub select HDD1 on BIOS and if you want to use Windows bootloader you just select the SSD1 from the BIOS menu (I’m reffering to the order sequence of booting).
You can replace USBdrive for HDD1 in the example.
Another option:
Let’s say you have SSD1 and HDD1. SSD1 has WindowsLoader and HDD1 grub.Then you install plopbootmanager into an USB (or CD?) and in the BIOS you select to boot from that USB. The plop will ask from where you want to boot and you select SSD1 or HDD1 alternatively.