How to empty trash quickly on MBA OS X Yosemite from External HDD?

I am using MB Air with OS X Yosemite. Deleted huge data (Pics / Videos approx 250GB) from my External HDD (MS DOS format). I tried secure empty but it doesnt work & gives an error (The operation can’t be completed because you don’t have permission to access some of the items). It shows up in trash whenever I connect Ext HDD. Please help.