Combining Devices to a RAID Array:
Now we must use the command mdadm to make a RAID device. Note that on Ubuntu this program is included in the the raid-tools package.

mdadm --create /dev/md10 --level=0 --raid-devices=2 /dev/disk/by-id/usb-0930_USB_Flash_Memory_0400AB6141E19A4C-0\:0-part1 /dev/disk/by-id/usb-0930_USB_Flash_Memory_0BF1FA61734179B2-0\:0-part1

I chose md10 as the device so there's little chance of it conflicting with any already existing raid devices. If you have more than 10 raid devices you probably know what you are doing!

I will make note and say that if you have dissimilar sizes of storage devices that you can set up multiple raid devices and use LVM to combine them into one.

Now that you have a raid device, you can check the speed of the device:

# hdparm -t /dev/md10
/dev/md10: Timing buffered disk reads: 74 MB in 3.08 seconds = 24.06 MB/sec

We get about what we expected--twice the speed as before. Until the usb bus becomes saturated with traffic, each device we add should add another 11MB/sec transfer speed.

Make the file system:
Next you need to put a file system on your raid device.

mkfs.ext3 /dev/md10

Note: If you have other raid devices on your system, be careful! Using the wrong device name can destroy your data!

Obviously, if you want a different file system than ext3, then use a different command. fat32 may have faster write time depending on the data. Keep in mind though, no matter what the file system type, you will not be able to use your raid from Windows. It simply doesn't provide the tools.