Or even better, try using 'screen' instead of 'minicom'. I gave up minicom a long time ago in favor of screen. Once you get used to it, you'll never go back!. Chris Stratton was probably right: you're overrunning the target. There is no flow control in virtually all embedded targets. That said, I do these copy operations from screen all the time using screen to set u-boot environment variables, and never have any issues. Depends on your target, too!
Invoke screen something like this:
screen /dev/ttyUSB0 115200
Of course, the device node corresponding to your serial port will be different depending on your serial device. The example above is typical for Linux with a USB serial dongle. On my macbook, it usually comes up as something like this for the same serial dongle:
/dev/tty.usbserial
If you like vi, you'll love screen.