Time Machine switch on backup harddrive
I know this is a novice question. Sorry all! I accidentally attached the wrong external harddrive to my old iMac (with new harrdrive) that I only direct Time Machine to store on. They looked the same. Now, I've put the original one back on it that the computer store had set up. Does the "wrong one" have more files saved on it than my original one that I just attached? Or is everything good to go now?
iMac 27″, macOS 10.13