As suspected, no problem accessing the onboard SD card slot with Python. Here's quick & dirty Python program that reads a text file from the card and writes it out to a new file on the card.
fileIn=open('/media/sdcard/input.csv','r')
fileOut=open('/media/sdcard/output.txt','w')
# infinite loop until EOF is reached
while 1:
line=fileIn.readline()
if not line: break
fileOut.writelines(line)
fileIn.close()
fileOut.close()
