On latest Linux kernel from SSD to USB or USB to USB with it's easy to lock up the copying application or even whole machine while data is flushed. It's less of an issue with deadline or bfq i/o schedulers, but it's still there. Now, we copy around at least gigabytes these days, but still. When I was using a Mac, the first thing I always did was to completely disable Spotlight (including all indexing).