Linux kernel priorities often assume that your memory and disk operations have priority over USB i/o. This leads to situations where you are waiting for some files to write and your system thinks you aren’t in a hurry.
This trick worked for me, I found it on one of Peter Stevenson’s github pages.
Copied text follows. Saving here in the blog for safekeeping! You will need “Sudo su” first to write the file. Cheers!!
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If your running a x64 bit Ubuntu or other Linux and find USB transfers hang at the end apply this fix:
echo $((16*1024*1024)) > /proc/sys/vm/dirty_background_bytes
echo $((48*1024*1024)) > /proc/sys/vm/dirty_bytes
I suggest you edit your /etc/rc.local file to make this change persistant across reboots.
sudo nano /etc/rc.local
Go to the bottom of the file and leave a space then paste in those two lines.
Save the file with ctrl + x then press y.
To revert the changes enter this in console and remove the lines in /etc/rc.local
echo 0 > /proc/sys/vm/dirty_background_bytes
echo 0 > /proc/sys/vm/dirty_bytes
More info and references: https://unix.stackexchange.com/questions/107703/why-is-my-pc-freezing-while-im-copying-a-file-to-a-pendrive/107722#107722