What you should be doing:
$this->bbcode_second_pass_code('', 'mkdir /tmp/alarm
mount /dev/sda1 /tmp/alarm
cd /tmp/alarm
wget http://archlinuxarm.org/os/ArchLinuxARM-kirkwood-latest.tar.gz
../bsdtar -xpf ArchLinuxARM-kirkwood-latest.tar.gz
sync
cd /tmp
umount /tmp/alarm')
and continue from step 11 in the official instructions
Note: ../bsdtar is no mistake here (2 dots, not 1).
What you are doing:
Probably logged out and back in between seesions or rebooted.
$this->bbcode_second_pass_code('', 'mkdir ~/alarm
cd ~/alarm
wget http://archlinuxarm.org/os/ArchLinuxARM-kirkwood-latest.tar.gz')
~ is a shortcut for home directory for current user which is likely to be tmpfs as well in stock OS
You can also be doing your way if you want, but you need to mount /dev/sda1 under ~/alarm then.