Previously I had backed up my Mac files on my old external hard drive on time machine. This made the hard drive only visible and usable by Time Machine. How would I reformat the drive so it goes back to FAT32 or something usable like a normal hard drive? The hard drive is not being recognised or detected by disk utility either.
If Disk Utility or the terminal
diskutil list shows no drive - you have a hardware or cabling issue.
Disk Utility or the command line equivalent is the correct way to erase / repartition / re edit (whatever you want to call it) any drive. They should show up as a connected device within 5 seconds of powering them on.
Once you’ve worked out hardware issues (or software if you have anti virus or other problems that are blocking the normal mounting of any connected drive) - then you can choose MBR/APS/GPT for the drive overall partition scheme and then build up whatever filesystem you choose.