OK. So I'm not as "gifted" when it comes to Linux as I could be. I freely admit that I have a long way to go. Yet, being a service guy (not only professionally, but of course, all my friends and family as well), I frequently find it useful to boot a troubled system using a non-indigenous drive and/or operating system.
While it's common to convert the "Live Linux" CDs (and DVDs) to USB, and set aside a portion of the USB drive for "persistent changes", I find it even more useful to do an actual install onto a USB drive. Up until Fedora 15, that was easy. There are a couple caveats I've noticed with F15, so I'll go through the whole process that I undertook in the hopes it may help someone else, and give them the tools that I have found so valuable.
To start, get a decent sized USB drive. While it's possible to install onto a 8G USB, a 16G is realistically the smallest that I find useful. This is because I typically store certain files on it that I frequently need to access (usually for fixing win-doze boxes).
First, make sure the box you're using has an appropriate DVD, and is capable of booting up via a USB-connected drive as well as the DVD! Power the box down and plug the target USB drive into a USB slot. If you REALLY want to be safe, go ahead and unplug the on-board hard drive(s). You won't be needing them.
Boot up on the F15 32-bit install media. (32-bit boots about anything, 64-bit is pretty exclusive)
With a few exceptions, the install routine will be identical to any other. Just remember the following;
1. Use the "BASIC VIDEO" option.
2. DON'T use Volume Groups. Partition the USB manually into primary partitions (ext4, swap).
3. (Maybe) If you haven't unplugged the on-board drives, DO NOT install a boot loader on them! MAKE SURE you select the USB drive! (trust me... you'll only want to make this error once in your lifetime!)
I used a 1024 MB partition for swap, a 512 MB partition for "/boot", and the rest for "/". Your mileage may vary, bit this works for me.
A couple other things you want to take into consideration....
When it gets to the package selection, I HIGHLY recommend that you GET RID of GDM and GNOME 3. Instead, select KDM and KDE. The reason for this should be obvious, but I'll go ahead and irritate "the community" and spell it out for everyone.
This is a TECHNICAL drive. It's meant for folks who are going to be fixing other computers (maybe even your own). It's NOT for the average user! You WILL need to log in as root, and you WILL need to be able to easily modify your desktop and menus to suit your convenience. While it only takes one line of editing in each of the files, /etc/pam.d/gdm and /etc/pam.d/gdm-password to enable root login, KDE seems to allow it by default, and is a LOT more "root function friendly".
Of course, if it's at all possible, during the install you'll want to use the update repositories. That way a minimum of space is used. I don't recommend using a wireless connection for this step though. Hard-wire the sucker...
During the install process, you'll have to reboot. Make sure to hit whatever keys are necessary to boot up via the USB. Continue booting off the USB until the install process is complete.
I also like to install a few things, and make a couple changes.
In "/etc/sysctl.conf", add the line "kernel.shmmax = 256000000 (OK, it's large... you can probably get by with something around the 132xxxxxx size, but if the box you're working on doesn't have it, they probably need a new one anyway...)
In "?etc/nsswitch.conf", change the "hosts:" line to "dns wins files". (your mileage may vary)
(note, some of that stuff getts installed as dependencies)
clamtk (also installs clam-av as a dependency)
fpscan ("FP-SCAN". gotta' get that from the site: http://www.f-prot.com/download/home_user/
avast (for linux workstations..... they don't seem to have a "paid licensed" version for anything but the Linux Server. It's not only expensive, it's simply not what's needed for this application.) Get that here: http://www.avast.com/en-us/linux-home-edition#tab4
nmap-frontend (also installs nmap as a dependency)
ntfsprogs (Should be there, but check. It includes things like "ntfsclone" "ntfsresize", etc... VERY handy!)
remmina (DAMNED handy when used in combination with openssh to get to your home system to retrieve a piece of software you forgot!)
Those are just a few, but the first ones I install. Maybe you really like "gdm". If so, edit the "/etc/pam.d/gdm" and "/etc/pam.d/gdm-password" files to allow root login (Just comment out the line pertaining to "root quiet" in each of those files). If you're fixing a system, "sudo" and "su" will probably be more of a pain than their worth, so just log in as root and be done with it! (...but be careful! Remember, in the Linux universe, root is god, and he CAN commit suicide!)
At any rate, you now should have an actual Linux installation that will boot on just about anything that can boot up on a USB. The advantage of this over the "Live" (but static) media is that you can add/remove software, perform updates, copy files, and do a host of things not possible with another type of O.S. The bootable CDs are still VERY useful, and I do take versions of them with me when I go on-site. The flexibility and reliability of a true Linux installation on a USB dongle is just amazing. If you haven't tried it yet, give it a shot. With a large enough USB drive, you'll even have room for a fat-32, or NTFS partition for files you can read directly under win-doze. All in all, I've found it VERY useful.
All the best.