Does your A500 detect disks in all those drives? I have read that sometimes with PC to Amiga floppy mods that while the drives work the Amiga does not automatically display what is in the drive like it does on a genuine Amiga drive.
Actually I don't really know but what I remember I have never noticed such behavior, but I can't be completely sure. Since I've been moving around lately, I have all my belongings in boxes. So I can't test it for you. But I can tell you that I could run them all att the same time. For an example: DF0: Formating, DF1: copy to DF2:, DF3: Virus scan. Plus a lot of other programs running (Powerpacker,scenery, ADpro etc.) just to demo the multitasking ability to friends.