In the latest version of the Router OS (starting from v.3.0.rc5) there is support for booting from USB devices. It means that you can install RouterOS on a USB flash/drive using CD-Install. It is even possible to attach the SUCB drive to a Windows computer and copy the RouterOS installation files using Netinstall to the USB drive.

This new feature allows you to test your PC and how it would work as router with a beta version or current version. It is also cheaper than using CF drives with adapters. It is a good option to run RouterOS from  USB stick on your laptop (usually booting to others OS) and use the advanced network utilities when analyzing a network

Installation

To install the RouterOS on the USB you can use CD-Install or Netinstall from the latest versions of the RouterOS (starting form v3.0rc5). Also it is possible to attach that USB drive to a Windows computer and copy the RouterOS installation files using Netinstall to the USB drive and when booting this USB drive it would install the RouterOS on it. Media Capabilities with Motherboards and Laptops. Since there are lot of USB media types, below are notes for most of that which have been tested.

USB Flash Stick

On Gigabyte GA-8I945P-G-RH motherboard that are using (945 chipset) you need to go in the BIOS and specify to boot the from the USB-HDD. If you have another hard drive installed in the same PC then you should also specify the boot priority. For the Gigabyte motherboard in the BIOS it gives two options regular harddrive that you have installed and Bootable add-in drives. You should specify the Bootable add-in cards. After the first reboot if it didn't boot from the usb drive, you need to reboot once more and go into the same section and there you will see a new boot option - the usb flash drive name.

USB Card Readers

Not only USB flash/drives can be uses, also USB card readers can be used. In these USB Card readers you can plug any media (CF, SD, xD, etc) and use that as a bootable device to run RouterOS. But there could be problems with these Card readers as sometimes it is not recognized as a bootable device in BIOS - such thing happened on the Intel DG965WH motherboard (965 chipset), but it worked on Gigabyte GA-8I945P-G-RH motherboard (945 chipset). It could be because of the motherboard BIOS and that shouldn't affect the regular USB flash/drives.

Additional Notes

The booting support for the USB drives is the part of the BIOS, so if your motherboards BIOS doesn't allow to boot from that drive you might try to upgrade the BIOS of the motherboard.