Why are there 2 folders SDCard and SDCard2 on a ViewSonic GTablet? I see that there's a folder named sdcard and another named sdcard2 on a ViewSonic GTablet.  If I put the SD card in my PC, the sdcard2 folder is recognized by the PC.  If I connect the tablet to the PC via a USB cable, the sdcard folder is recognized on the PC.  Neither way allows me to copy any file to the root folder of the tablet (i.e. the folder named /).

Please explain as I'm kind of lost.
According to this thread /sdcard is the internal storage of your device. It is so named because Android counts on a FAT/FAT32 filesystem at that mountpoint for data storage. The optional external sdcard mounts to /sdcard2 and is evidently not available via USB from the tablet. These are the "user serviceable" locations in Android. To work with files outside these paths you will need to have root access on the device and use a 3rd party file explorer.