[ubuntu] 12.04.1 installation question

Hello,
first, I have a disk(A) with ubuntu 10.10+GRUB installed,
and I want to install ubuntu 12.04.1 on another disk(B,without any operating system), unluckly, I don't have cdrom and the computer mainboard does not support boot from usb.

so, I install the ubuntu 12.04.1 by this way:
1. download precise iso from (seems only alternative version works) http://releases.ubuntu.com/precise/u...rnate-i386.iso
(md5 and sha1 sum both checked)

2. extract iso:/install/initrd.gz and iso:/install/vmlinuz
to diskA:/boot/ and add a menuentry in the diskA:/boot/grub/grub.cfg, then I can boot into the setup wizard.

3. when setup wizard detects cdrom, I enter another console(Alt key +F2), and exec:
mkdir /t
mount /dev/sda2 /t
#/dev/sda2 is a partion of diskA
mount -o loop /t/ubuntu-12.04.1-alternate-i386.iso /cdrom

I once did this sucessfully and boot into the new ubuntu 12.04.1,
yes, from diskB, however, after sometimes later, I did the same step again, but can not success because when I run:
mount -o loop /t/ubuntu-12.04.1-alternate-i386.iso /cdrom
I always got:
mount:mouting /dev/loop0 on /cdrom/ failed:Invalid argument

I was stuck because I did installation successfully once, but why failed after ?

Thanks for your help!