Edit: this post assumes a PS-04...
if you program a whole lot of drum patterns - can you copy them from one card to another via a computer?
Sure. No problem.
Pop the memory (SM or XD) in the card reader, copy over any and all files to a folder of their own. Name it by the name of the song so you don't lose anything and can work on it later.
By using the card reader, you can save individual tracks, save entire projects, leap small buildings with a single bound (strike that).
MAC_PRM.INI, PRJECT00.INI, RHYTHM00.SEQ, plus any .AUD files associated with the project such as 01-00.AUD 01-10.AUD, etc. are the files for a project with two tracks, plus FASTDT00.FST, if you used the F.A.S.T. method for making your pattern.
I have a blank project in a folder on my computer with no .AUD files, just the others above (MAC_PRM.INI, PRJECT00.INI, RHYTHM00.SEQ, FASTDT00.FST), and I import it to a newly formatted card to start a project.
As you probably know, after you format the card, the machine is ready to record on project 00, but you have to go in and turn on the LCD backlight and set your effect patch. By importing the stock project, I save a few seconds doing that, plus I have imported any custom stuff I spent time programming.