Dry chemical extinguishers (ABC) use corrosive/reactive chemicals (eg monoammonium phosphate etc.). The fire is out but the chemical gets into places you don't want it and it will cause future problems, electronics etc. Plus it can be a mess to clean up just on surfaces. Sort of putting out a small fire creates collateral damage beyond what the fire caused.
A CO2 extinguisher is less of a problem in this context. I recommend though that you have both, put the CO2 at closer reach and have the ABC as a back-up to save the building/lives if the CO2 does not get the job done. CO2 only if the area is large enough not to snuff yourself out and for a fire that is small and just starting, shed or small one car garage are likely not a great idea, stick with ABC.
Also think about location and exits, etc.
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Locate them at the exits and always stay between fire and the exit when trying to put anything out (shoudl be common sense...). Many documented DIYer cases where people went to the back of the garage with no exit to grab the extinguisher only to be found later by the fire department dead holding an empty one (that they emptied trying to fight the fire).