I want to
send a 150k file over Serial Xbee from the SD card ON ARDUINO UNO
I think it would be helpful if you explained why. Is the file being written to on the Arduino?
If so, why is all the data being put in one monstrous file? Using several smaller files, like a new file per day, would make more sense.
If not, simply removing the SD card and copying the contents on a faster computer would make more sense.
Does the speed at which the transfer takes place matter? If not, sending and acknowledging each packet would enable you to send the whole file with certainty that the data was received correctly.