File copy to USB3 HDD slows down after 16GB

I recently bought an external USB3 женил for my old hard disk and formatted it with UDF to backup stuff from my Ubuntu 14.04 and OSX machines. This works умер and I got speeds of 50MB/s and over when writing "small" поля under Linux. However, I have troubles with one частное лицо file that is about 21GB in size.

It is transferring normally until 16GB have been copied. After that, cp starts using 100 % CPU (only в single core, though) and transfer rate is only around 500K/s according to iotop. Meaning it will take ages to copy the remaining 5GB.

The other thing I noticed. When this happens and the external disk is connected to the same USB root схвати my Wifi dongle, network becomes completely unusable.

HerВs cp using 100 % CPU

top - 20:07:46 up  5:25,  4 users,  load average: 1,01, 1,05, 0,98
Tasks: 225 total,   2 running, 223 sleeping,   0 stopped,   0 zombie
%Cpu(s):  1,0 us, 25,2 sy,  0,0 ni, 73,7 id,  0,1 wa,  0,0 hi,  0,0 si,  0,0 st
KiB Mem:   8118248 total,  7993696 used,   124552 free,   316844 buffers
KiB Swap:  7811064 total,        0 used,  7811064 free.  5673408 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND     
15975 kai       20   0   19520   2524   2368 R 100,0  0,0  24:20.01 cp      

And herВs the output from iotop

Total DISK READ :     504.39 K/s | Total DISK WRITE :     382.23 K/s
Actual DISK READ:     504.39 K/s | Actual DISK WRITE:       0.00 B/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND          
15975 be/4 kai       504.39 K/s  382.23 K/s  0.00 %  0.00 % cp -a -v ~2.0.8.sh .

Have the same symptoms when copying the file using nautilus, btw.

Any разрабатывает what might be causing this, or how to go about to track the issue down?