I did figure it out somewhat, but it doesn't work well unless you've got usb data storage.
Note: In stepmania.ini i set all the usb variables to -1 to make a portable hard drive work. Otherwise it'll go for the usb drives by default instead of the OsMount variable.
I was using a PCMCIA hard drive (portable hard drive) and stepmania would only accept it if it was in when stepmania loaded. In other words, I couldn't switch hard drives or switch save files. I'm guessing you can only switch with usb hard drives because it's set to autodetect those. I also noticed, when the variable was set to usb, It said insert card instead of loading my PCMCIA hard drive. I'll post again once I get a card reader.
As for the pen drives you'll probably have to set the mount drives to the first available unused drive. If your letters go to E:, then make P1 as F: and P2 as G:. I'm pretty sure windows just uses the next available letter so P1 will have to insert theirs, and then after P2 will insert theirs.
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