Hi there EpicTheron,
First of all, you may want to back up your most important data stored on the failing drive. Just copy it to a flash drive or some kind of a cloud.
Then, you can consider cloning the failing drive onto the spare one. You will need a SATA to USB adapter. There are many threads on cloning in the community. You can use a brand specific tool (most of the HDD manufacturers have their own cloning tool) or use a third party one. This would create exactly the same copy of your old drive and put it onto the new one. Then you just switch the drives.
The process should look like this: Back up the important data stored on your failing drive -> attach the spare one via SATA to USB -> back up all the data stored on the spare...