What phone are you using? I wonder if one is internal memory and one is your sdcard. Check the sizes of both volumes, the one that matches your sdcard size is, obviously, your sdcard.
Turn off your phone, before pulling out your card when you check the size label thats printed on the card.
4 or 8GB are pretty much the standard size cards that ship with phones.
To verify, your plugging in your phone to the mac with the usb cable, then pulling down the notification bar and mounting the phone to your computer?
The other option is to turn off the phone and remove the card and put the card into a card reader if you have one. If your like me you'll have a ziplock bag just full of sdcards and accessories
**edit** I see you have the incredible, and I looked and some people are having problems with win 7 and the included sync program. (?) The safest/fastest thing to do is to use a card reader, i guess, and bypass the phone.