"Missing Operating System" when trying to boot from USB

Question

I have a macbook air (3,2), and I'm trying to install Arch onto the partition that currently has Ubuntu. I have tried a few different ways to get the Arch core iso onto the usb key drive, but no matter what I do, I am ending up with the message "Missing Operating System" when I boot into it.
I started out using the instructions from https://wiki.archlinux.org/index.php/US … tion_Media for writing it on OSX, using diskutil unMount, then the dd with bs=8192, and of=/dev/disk2
When that didn't work, I tried booting into my ubuntu install, and used just a plain
sudo dd if=archlinux.iso of=/dev/sdb
Still no luck, so then I went to my old standby of unetbootin, which didn't do any better. I just tried yet again with the dd in ubuntu, and I'm still not having any luck. 
Here's some of the relevant info I could find:
$ sudo dd if=Downloads/archlinux-2011.08.19-core-x86_64.iso of=/dev/sdb
772096+0 records in
772096+0 records out
395313152 bytes (395 MB) copied, 1.55667 s, 254 MB/s
$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 8065 MB, 8065646080 bytes
64 heads, 32 sectors/track, 7691 cylinders, total 15753215 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x3946b2c9
   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           0      772095      386048   17  Hidden HPFS/NTFS
One interesting thing I noticed was that the dd took about 10 minutes in mac osx, and under 2 seconds in linux.  That seemed like a pretty big difference between the two systems, but I'm not sure if that's related to the batch size on the write, or to the implementation of dd used in OSX.
Is there something obvious I'm missing?  I know the computer is able to boot from this key drive because I was able to boot an Ubuntu livecd from it just a few days ago.  Unfortunately, the MBA won't boot from my external cd/dvd drive because it's not the apple brand super-drive, and I don't want to spend the money for a super-drive, so a cd-based install isn't an option.
Really looking forward to playing with Arch, because some of my friends recommended it to me highly. I'm a former gentooer (years ago) who became fed-up with installing *everything* from source. I can't stand ubuntu doing everything for me, and debian always seemed so far behind the curve on everything, so Arch seems like the perfect OS if I can just get into the installer. :)
Thanks so much!
~Maquis

Answer 1

Oh, forgot to mention, I did the dd from ubuntu with both bs=4M and without a bs argument. the version without the bs argument is the one that went really fast. 
Neither version would boot, so I don't think that the way I'm writing it is the problem.
I did just notice that if I just do a sudo fdisk -l without specifying the drive, I have 2 entries for my drive:
Disk /dev/sdb: 8065 MB, 8065646080 bytes
64 heads, 32 sectors/track, 7691 cylinders, total 15753215 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x3946b2c9
   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           0      772095      386048   17  Hidden HPFS/NTFS
Disk /dev/sdb1: 395 MB, 395313152 bytes
64 heads, 32 sectors/track, 377 cylinders, total 772096 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x3946b2c9
     Device Boot      Start         End      Blocks   Id  System
/dev/sdb1p1   *           0      772095      386048   17  Hidden HPFS/NTFS