i THINK (i could be wrong...) a pendrive would require more nearer 5volts to work...
opened up my SDcard reader a few days ago, plugged into usb with a 2gb sdcard...it was pulling 3.4-3.6volts while accessing the card, which is damn close to the MicroSD maximum voltage...
MDA Vario III tmobile UK -Radio: 1.64.08.21 -Rom: 3.0 sleuth WWE - 2GB Sandisk
and lots of customisation! now resprayed flourescent orange!
HD2 tmobile UK - ROM: 126.96.36.199 (86525)WWE - RADIO: 2.07.51.22_2 - 8GB kingston
Desire HD - Rooted/CWM/android revolution 4.0/32GB storage and loving it!