Grub-rescue
 

I have installed a new hard drive on my acer 5920 laptop. I have installed windows 7 and linux 12.04. The other hard drive is in an external drive unit and when I tried to load via usb I got the message Grub Rescue. Stupidly I did not back up the data and would like to retrieve it. Can anybody help?

Are you just transfering the operating system(s) from 1 drive to another?
If so try clonezilla-live-cd/usb.
http://clonezilla.org/
http://clonezilla.org/clonezilla-live.php

What exactly are you trying to do? You got a new hard drive so, did you then install windows 7 and Ubuntu 12.04 on this new hard drive? What other hard drive? Did you install windows 7 and Ubuntu 12.04 to the "other" external hard drive? or to the laptop internal? Are you trying to boot the external hard drive? If you have some operating system and not just data on the external and are trying to boot it, where is the bootloader? The mbr of the internal, the external mbr, the root partition?

Grub Rescue
 

I have installed ubuntu 12.04 and windows 7 on the new hard drive. The hard drive removed is now in an external drive unit and when I try to load from the external unit I get the Grub Rescue error message.

Your old drive is configured as a primary boot device, with it's own grub installed,... that's the issue...

Grub is seeing two primary boot partitions on the system, with notthing to make sense of them, and is freaking out because of it. If you migrated your data already, you can wipe the drive. It should not be mounted when you do this. It'll show as something like sdb (instead of sda). If you need stuff on it you might have to uninstall grub from that drive. Grub commands are a P.I.T.A. If you can avoid having to deal with them, just wipe the drive.

Trying to access the old, installed systems on the drive may cause a problem,... wading through all the mess for what you want... If you don't need anything on it,... fdisk it...

By the way,... If the drive is in an external enclosure, and powers up separately,... have you tried bringing up the system first,... then the external drive afterwards, so it doesn't interefere with the boot process? You should then be able to have it seen, but not mounted, and should then be able to repartition it.

At any rate,... When migrating to a new HD, it is best to have an external backup drive available to use for migrating data. You just migrate your files to backup, disconnect that backup drive, reboot the system and delete all original internal partitions. Power down. Replace the hard drive. Install new. Attach your external drive and migrate your data back. Then, you can do whatever you want with the old drive.

In your scenario,... try booting from something like Gparted Live or Parted Magic,... From there make sure to correctly ID your disk volumes (so you don't delete your new install). Use the Live CD to move data that you need,... then use the partitioning tools to remove grub from the external drive... or repartition, or whatever...

Oh,... Alternatively,... you could always put your old drive back... backup your data,... VERIFY your backup,... Delete your partitions entirely (Gparted or Parted Magic). Re-install your new drive,... and then... back on your merry way with the old drive in the external enclosure?!?!

You can either mount the drive from a running os & recover data, or, use clonezilla to copy to clone os to the drive, you do not try to boot the external drive, windows will not agree with you.

Although my debian wheezy installation was once sda it boots & I didn't make any changes to it.