There's an issue with Ubuntu that hasn't been fixed yet, where the PC freezes or gets really slow whenever it is copying to an USB stick (see Why is my PC freezing while I'm copying a file to a pendrive?, http://lwn.net/Articles/572911/ and https://askubuntu.com/q/508108/234374).
A workaround is to execute the following commands as root (see here for an explanation) as root:
echo $((16*1024*1024)) > /proc/sys/vm/dirty_background_bytes echo $((48*1024*1024)) > /proc/sys/vm/dirty_bytes
How do I revert these changes? When I restart my PC, will it get rolled back to default values?