The latest DIOS MIOS is 2.10 (different than 2.1). So make sure you have the latest. Any of the three current loaders can do this. There isn't a good "stand-alone GC laucher". I use CFG, personally, but any of the three will work.
Check the Dios Mios guide to make sure that are properly setting up the game images with DMToolbox and installing them to the hard drive correctly. Or use GX or CFG to rip the gamecube disk directly to the hard drive.
Also make sure the hard drive is FAT32 with 32k clusters (guide will show you that as well).
Links to all guides and apps mentioned exist in my signature.