First, I think you need to re-do the jumper settings. Then we'll get to accessing.
From your description I am assuming that the old HDD is an IDE type with a wide data ribbon cable containing 80 wires. But it has 3 connectors on the ribbon, and they each have only 40 (or 39) holes in them. There ought to be a Blue connector on one end, and that goes to the mobo IDE port. There ought to be a Black connector on the other end, and a Grey connector in the middle.
IDE devices have jumpers and pins that need to be set to establish Master or Slave. SATA devices do NOT - they may have jumpers and pins, but for different reasons entirely, and most often you should NOT change the jumpers on a SATA device unless you know why and how. Now back to IDE devices.
Any IDE port can support up to two devices, and hence each needs a unique way to identify it. That is what the Master and Slave settings are for. They are relevant ONLY to IDE ports, and ONLY to the ONE port being shared. There is no such thing as one Master device for the whole machine, with a number of Slaves. In the machine there IS one (usually, but it could be more) BOOT device, and other data devices.
On ONE IDE port you could have one or two devices sharing the port and data ribbon cable. The first one MUST have its jumper set to Master. On some HDD's there is a different setting for Master with no other device, and Master with Slave Present, so set this according to the use of THIS ONE port only. The Master device should be connected to the Black END connector of the ribbon cable. IF you also have a second device to plug into that same cable and port, then its jumper MUST be set to Slave. (If necessary, the Master device's jumpers may need a slight adjustment to the "Slave Present" setting.) This Slave device should be connected to the Grey MIDDLE connector of the cable. IF you have both an optical drive and a hard drive sharing one IDE port/cable, it is recommended that the HDD be the port Master, and the optical drive the Slave.
As an alternative, you CAN set the jumpers on BOTH devices to "CS" (for "Cable Select"); then the one on the END of the cable WILL be the Master, and the middle one WILL be the Slave.
Note that, if you have two IDE devices and two IDE ports/cables, and you connect one device to each port, then BOTH devices MUST have their jumpers to Master, because each is the Master of the one port it is connected to.
Now, if you set up your IDE drives this way they should all work fine. The BIOS should detect them properly (assuming that your IDE ports are Enabled), and Windows should also "see" them as valid hardware devices, assigning them letter names and showing them in My Computer. Your post indicates that did happen up to Windows' giving it a letter name. How do you know that? Where do you see this info?
However, your post indicates that you can't access its files. That is what I don't get. If it has a letter name it ought to be showing in My Computer. Is it? What happens when you click on that drive there? Does it not show you the files and folders?