Hardware copiers are so called because of the piece of hardware that fitted in between a computers disk drives. Some examples are Cyclone on the Commodore Amiga, or Synchro Express on the Atari ST.
Disks copied with these devices are actually only an “approximation” of the real disk. The devices themselves are rather analogue in nature, and thus never produce a 100% copy, even though the game may indeed work. There were types of Copy Protection that could easily fool these devices though, and would not work at all when copied.
You can copy a game with these analogue copiers directly from an original disk, however that copy will normally fail. Unlike the marketing the companies doing such analogue devices hint at, this has nothing to do with preventing copies of copies (”generational copies”) for piracy reasons. It is just that the bit cells get so distorted after copying that they will not normally hold together on a second or third generation.
To get the idea, think of copying a VHS tape and then copy that copy, and so on... The quality degrades, but you can still watch the content. Obviously if the quality “degrades” (the data changes) for a medium of a digital nature, i.e. graphics data, sound data, compressed data, or program code, your program will fail to some greater or lesser degree that may not be evident from casual use. For graphics and sound you may get a missing effect; garbled or missing graphics; for program code you will probably get crashes; for compressed data memory corruption may occur and any or all of the above - depending on the content compressed.
There is another reason though, when the disks were commercially duplicated they are written very “neatly”. We can see when a disk, or any tracks on a disk is written by an Amiga because they are not written as “neatly” and this is one of the things we use to tell when a disk has been modified. Hardware copies are even less “neat” than a normal Amiga write, depending on how well the software was able to synchronise the two drives doing the copy. Because of this there is no way to tell if the data was modified before the copy or not.