Cannot access external HDD

Hello. I am very new to this and only slightly :P computer literate. I have the following issue.

My old desktop's (Windows XP) motherboard died. I removed the HDD and attatched it to my laptop (Windows 7) via a SATA to USB 2.0 cable. My laptop reads the HDD and shows the memory about half full but shows no files. I downloaded Recuva to attempt to find these files. The now external HDD is listed under the Z: location. When attempting to use Recuva to access this I have the following issues...

If I ask it to scan the Z: location it states it is an "Invalid drive for scan."

If I ask it to search as a media card or iPod, it states "No drives were scanned,because none matches with the filter."

If I ask it to search the entire computer, it will scan the laptop but no files appear under the Z: location.

Is there a solution to this using Recuva? Is my entire attempt to recover this data flawed? Is there any solution?

I am mostly doing this to attempt to recover music. There is enough there that I would be willing to pay a reasonable amount of money to fix it but I hear trying to get a professional service to recover the data on the old HDD could cost hundreds which is not an option for me.

I will happily supply any specifications that would be needed to help me resolve this. Thank you very much in advance for any and all help in this.

First of all, let's backstep a bit.

Your old motherboard died and the hard drive from the old system is now USB attached to your laptop.

There should be no need for Recuva as nothing had happened to the old drive.

Unless you Mapped a Network Drive, I can almost guarantee Z:\ is not the old PC hard drive. (maybe the external enclosure or cable you have it in has done something, but not in my experience)

With the old drive plugged in, simply go to My Computer, down the left side in the Folders Pane, you should see A:\ if you have a floppy, C:\ for your laptop hard drive, D:\ for a second partition of C:\ or your DVD drive and so on.

Windows keeps assigning the next letter to each successive drive, so your external drive should be E:\, F:\ or G:\ a the most - depending on if you have USB memory sticks plugged in, or USB printers with memory card readers in them etc.

This is the drive letter you need to look for, not Z:\