This Arch-wiki page has a lot of related goodness laying out different approaches to making one. Bunch of grub entry examples for a metric ton of diff distro's and refers to a couple Github projects, which are supposed to make it really easy to make and manage a multiboot usb. Finding the right entry to add to grub-legacy or grub2 has caused me and no doubt many a nixer to pull out a handful of hair.
This helped me to better understand the usage of gdisk.
And along the way, went ahead and installed rEFInd to be my UEFI/GPT bootmanager. Gives instructions for installing on several varieties of gnu/Nix. Pretty straight forward for whichever someone is using. I really like the thing, gives a good looking boot menu screen, plenty of themes someone can install etc etc.Edit: Though is kinda sparse in a few key ways too imo. Such as how to fire up gdisk.
Run lsblk, mount ... whichever command to find your usb device. In the below example am using a usb which is showing up as sdb. Simple enough to get gdisk to work on it, as root just type. "gdisk /dev/sdb" of course without the quotes and it'll fire up asking for further commands.
Now the backstory babble. Finally decided to get rid of window$ on the pc. Window$ was sitting on 100gbs of diskspace, hardly ever use it and my feelings about Microsoft are well babbled about. Thought screw it time to get rid of their software. So that's what I did, formatted the win8.1 partition and reclaimed my diskspace for something I actually want on the system.
Of course being a dork, wasn't enough. Was also left with a MSR = microsoft reserved partition and a lil 400mb partition at the front of the drive, which is-was the WinRE = Win recovery environment that used to go to an OEM recovery partition I'd long since deleted. So biz went into delete partition frenzy mode.
Wham MSR gone, Wham winRE gone and after backing up the Nix related stuff on the systems ESP/efi partition WHAM the ESP partition too. Oops, that's where things got a lil hairy. Should've been a simple matter of just setting up a new one and marking it with the boot flag, copying over the backups and called it good.
Screwed this up and left me with a system saying, "please install an operating system." Arghhhhhh, was thinking I'd borked the disks GPT table and for awhile sweating bullets. Finally just resorted to installing another LM OS and that recovered things enough to boot w/o having to chroot any of the OS's with live media.
Remembered rEFInd and went ahead and installed it, overwrote the temp LM install and everything was/is back to running happily. Though after dorking with rEFInd and liking the thing thought I'd share and mention it at the Grill. It's a pretty cool lil piece of software. One thing I found and like, it installs icons on the boot menu screen. Use the arrow keys to go back/forth, press Enter to select and when one is highlighted and you have several kernels installed for that one/OS, press the F2 key and it lets you select whichever you want to boot.
Second nix adventure: Been wanting to try Kali Linux for a long time. Downloaded their latest release, chose the light variety, it's 1.1gbs. Go to make an install usb with the dd command anddddd hell no, over and over just won't work. Still haven't managed it but this got me onto the whole multiboot usb project. Making definite progress, the usb is booting in full UEFI/GPT mode and atm, goes directly to a grub> rescue screen. Because I don't have any .iso's in the usb's data partition yet. That's planned for later.
Am not planning on using one of those Github projs. It's now become a personal thing and I want to do it by hand but they looked good and the Arch-wiki reference is pretty dang rich in good info and the gdisk link helped me learn to use the util. So again ... just decided to share and babble some about it.