You have a 1200, so you should have crossdos as part of the operating system. If so you can format a double density floppy as a 720k ms dos compatible disk. A windows pc cannot read an Amiga 880k disk.
To see if you have crossdos, open up your system disk (DH0) and look in Devs/dosdrivers. You should have a file called pc0. If it's not there look in the folder called storage. If it's there copy Pc0 to the Devs folder and reboot.
A much better explanation is here:
https://wiki.amigaos.net/wiki/AmigaOS_Manual:_Workbench_CrossDOS
