The U.S. Doubler lacks enough RAM to access PC disks. You need at least 512 bytes of RAM to write PC sectors, but it has only 256 bytes.
In theory you could read (but not write) PC disks reading the same sector multiple times. But this would require extra code that is not present in the U.S.Doubler ROM.
The Happy doesn't have that code in ROM either. But it has plenty of RAM so you can upload extra code to the drive at run-time. Something that you can't in the U.S.D.
The Happy was made in the US, not in Germany (don't worry I'm not from the US neither). There were plenty of ilegal Happy clones made almost everywhere though.
