I have ALL my image files stored on an external USB2 hard drive and Photo Manager sees and can index them without any problem. So, this is NOT a problem with the software, but rather something specific to your system.-, I'm afraid.
Can you say rather more detail about what you mean when you say you select it but it will not attach. Especially, what do you mean by the word "attach"?
First, thank you for reposting this to your original thread!
Second, don't use the Import function. That is for direct importing of photos from a camera.
Third, do the following:
Make sure you are in Advanced Mode, a tick box at the upper right hand corner of the main workscreen:
Then the left hand column of the workscreen will look like this:
Click on "Folder". This will then open a dropdown that, on my machine, having selected the appropriate drive, looks like this:
Select the folder or folders containing the pictures and let PM index them for its database. (This may take quite some time, depending on how many pictures you have.)