You have the MacPro4,1. You should tinker with it more to familiarize yourself with its internals. It should be very easy to open.
If both your hard drives are in the drive bay, you should be able to see the ones that are occupied. You can pull them out so you can read the make and capacity of the drives on the labels. If you don’t care about the contents of the “bad” drive, Disk Utility should be able to erase and run First Aid on it. For all you know, you might have a still-functioning drive.
Your SSD could be in a third drive bay. There are adapters for installing SSDs to the Mac Pro’s drive bay. If it’s not in one of the four bays, look at your SATA slots. There are also SATA SSD adapters. I have two of them in my MacPro5,1 (Mid 2010).
By the way, I’m getting ready to bring my MacPro to an Apple store to be recycled. Since you are a MacPro user, If you happen to be in my vicinity (Mountain View, CA) you are welcome to take it off my hands. There are no hard drives or SSDs because I moved all of them to a couple of Thunderbolt 3 external enclosures attached to my Mac Mini. It still has all the memory modules (32 GB?), and the graphics card was updated to metal in order to run High Sierra, or was it Mojave? Probably the latter because I don’t remember updating the macOS in the Mini when I got it in 2018 with Mojave installed. And two optical drives (DVD and BD?), not sure now. I haven’t used it since 2018.
Anyone interested? No strings.