Make disk/disk copy slower

You can throttle a pipe with pv -qL (or cstream -t provides similar functionality)

tar -cf - . | pv -q -L 8192 | tar -C /your/usb -xvf -

-q removes stderr progress reporting.

The -L limit is in bytes.

More about the --rate-limit/-L flag from the man pv:

-L RATE, --rate-limit RATE

    Limit the transfer to a maximum of RATE bytes per second.
    A suffix of "k", "m", "g", or "t" can be added to denote
    kilobytes (*1024), megabytes, and so on.

This answer originally pointed to throttle but that project is no longer available so has slipped out of some package systems.

Instead of cp -a /foo /bar you can also use rsync and limit the bandwidth as you need.

From the rsync's manual:

--bwlimit=KBPS

limit I/O bandwidth; KBytes per second

So, the actuall command, also showing the progress, would look like this:

rsync -av --bwlimit=100 --progress /foo /bar

I would assume you are trying not to disrupt other activity. Recent versions of Linux include ionice which does allow you to control the scheduling of IO.

Besides allowing various priorities, there is an additional option to limit IO to times when the disk is otherwise idle. The command man ionice will display the documentation.

Try copying the file using a command like:

ionice -c 3 cp largefile /new/directory

If the two directories are on the same device you may find linking the file does what you want. If you are copying for backup purposes, do not use this option. ln is extremely fast as the file itself does not get copied. Try:

ln largefile /new/directory

Or if you just want to access it from a directory on a different device try:

ln -s largefile /new/directory