[Roy Dennis]

It is probably because Hoster doesn't know the new path of where the files are stored.
When you plug a new drive in it is allocated a volume Directory letter. If you copied these files from an existing external hard drive to this new external drive then it would have to have a different volume Directory letter than the one you copied from so if you want to run from this drive you either have to set it's volume Directory to the same as the one you copied from or re-add it to the folders list in the Build database dialog using the add Folders or Add folders/Sub folders button.
If the files were copied from the C:/ drive then you can't make it a C/ drive as well, but if you copied from another external then you can set it's directory to the same providing it is not connected at the same time.
To change it's directory path to match that showing in the list of folders note the directory letter used there.
Now open control panel look for "Administrative Tools"then Computer management/Disc Management in this dialog you should see your drives appear in the bottom window, right click on your new drive and in the window that appears select "Change Drive letter & paths" another window appears click change, then select a volume letter to match that of your previous one then OK.
This will only work providing you don't have both external drives connected at the same time

If you are still having problems then give us more info like operating systemXP, Vista or windows 7 and the external volume letters.
If you choose to use the add folders option, you will have to remove all the existing folders from the list in the rebuild dialog otherwise Hoster will regard them as duplicated files and not load them.