You just need a 720 KB double density PC-style (FAT) formatted disk. It seems your Amiga is already set up to use it. The Amiga can't read/write high density 1440 KB disks.
You can recognize DD disks for not having a hole opposite the write protection hole with the slider. You can use any Amiga disk and reformat it on the PC.
Since it is an external floppy drive and not the original internal floppy drive inside the A1200, it could have possibly been a HD 1.44mb floppy disk drive, but probably not, as they are fairly rare (I have one though, the slim one made by Dell for the Amiga).
I would have suggested to format it using Cross-DOS on the Amiga and then put it into the PC to see if Windows could read it, by clicking on the PC1 icon and choosing format in the menu, but that is just me.
