I suspect that your new internal drive has assigned a different letter to the external drive. Look at the drive letter that LR is searching for and look at the drive letter that the new install on your new internal drive has given your old external drive. If they are different, then you have two choices. First you can re-assign the drive letter that LR is searching for to your external drive. This is done through the Computer Management window. God to Disk Management under the Storage tab and that is where you can change assigned drive letters. If you are not familiar with how this is done, ask for more detailed instructions before proceeding. Alternately, you can tell LR where the files are by pointing it to the external drive. It can then update the location of the images. If this is not the case, please provide a bit more details on the situation.
--Ken
