What I'm seeing is a folder/directory, not a button. It shows up under /mnt in three different file managers.
I don't have the device with me, but there were several folders under /mnt. The ones that I recall from memory are
Originally Posted by Mrhelper
The standard mount points (under /mnt) on the a500 are shown below. /mnt/usb_storage is used by the OS for any USB storage device. The /mnt/sdcard mount point is used for the internal storage, and /mnt/external_sd is used for the external SD card. Maybe the file manager you are using has a button named usbdrive intended for some other purpose. Just a guess.
app_102@android:/ $ ls -l /mnt
drwxr-xr-x root system 2012-08-11 14:24 asec
d---rwxr-x system sdcard_rw 1969-12-31 19:00 external_sd
drwxr-xr-x root system 2012-08-11 14:24 obb
drwxrwxr-x root sdcard_rw 2012-08-09 23:34 sdcard
drwx------ root root 2012-08-11 14:24 secure
d--------- system system 2012-08-11 14:24 usb_storage