I'm new to working with and using a backdoor/virus with a boot disk.
- Can they be mounted to a USB drive?
- What options are out there and what should I look for in one?
- Are they practical?
- Do they need to be upgraded/replaced every few months?
If your machine can boot to USB, then you can use an image designed for USB booting, but most are designed for CD/DVD
Just about every AV vendor has a live CD to boot from. Look for one that can access up-to-date virus definitions.
They are very practical from the standpoint that they can inspect the disk while it is at rest and not running an operating system. There are still types of malware that can hide from offline scanning, but live CD's are a real option for most types of malware. On the other hand, in my experience, live CD's can be extremely slow, which reduces their practicality for regular scanning.
Check out YUMI and XBoot at: http://www.pendrivelinux.com/tag/multiboot-live-usb/
These will let you boot many CD/DVD ISOs from a USB stick. Not every ISO works, but pretty much anything that is Linux based (as most bootable antivirus disks are) will work, and some PE based disks will work as well.