If your external HD is corrupted and not readable, then you will have to find your image file backups and put them on the new HD using the OS Finder/Explorer file copy.
Your images are NOT in Lightroom! (it is only a database!) You cannot restore images from the corrupt drive to a new drive using Lightroom if the drive is not readable.
Maybe file recovery software can find images on the corrupt HD.
This is what I was thinking..my big issue is then that all edits will be lost correct? as after I move the image files I will have to reimport they into Lightrooms library ..By the way I am using a Mac
The edits are lost only if you cannot point LR at a good copy of your images.
If you made a backup of your external drive, you can copy that backup to your new drive; then, as long as the folder structure is the same as the old drive, you just point LR to the top-level folder(s) of the new drive in the Folders panel, and LR will reconnect everything for you.
OK thanks! I was just trying that with one folder and it did seem to work..yes the folder structure will be the same as I will be moving the top level folder. Its a bit strange but good I think as bad drive is readable but Disk Utilities says it can only be backed up (files moved) but can not save changes to the files..so using Finder I can move the whole file structure and then just point Lightroom to them..Thanks again. Over 500G of pictures.