Unless you're working on files directly on the USB drive (why?), just plug it in, transfer the files, unplug and keep charging. In the off chance that you're really down to the end of your battery and need a charge, Macs will go into deep sleep mode when they hit 5% battery, which should be plenty of reserve battery power to do a small file transfer to/from a fast USB flash drive and then continue charging right away. You're making up a problem where there isn't one.
4/28/15 at 11:59am