I do similar. I have multiple folders on my SD card, one for each firmware version. The individual files of the firmware I currently want to run is on the root directory. This has worked flawlessly for me.
But I do not recommend it unless you are really comfortable with directory structure and moving files around.
pmotz said
I bought some Sandisk standard (blue) 2 GB cards from Amazon a year or so ago, which worked without formatting. The price was below $10 each. I just checked and they have them now for $19.99, ouch! Guess they are becoming scarce. I'll have to stock up on them and sell them for big money in a year or two!
The SanDisk 2GB cards have always worked for me on my P10 Power Plants, PWD, and now DS with 1.2.1 and PP FW upgrades. I never needed to reformat. After I saw your post here, I ordered 2x2 packs yesterday from Amazon for $35.49 not to take any chances. I used to be able to buy them locally at about $10 each, but they have been discontined everywhere it seems.