Did you try more than one flash drive? sometimes a flash drive is not correctly formatted, and the Stereo can't recognize it. Also examine the files on your flash drive by inserting it into a computer, go to a music files - Then go to "Properties." This will let you know if the files are actually MP3. Just some suggestions.
2006 GT, Strut Tower Brace, Gas Hood Struts, FR Intake Plenum cover, Metal Fuse Box cover, Moroso brake reservoir cover, Rear shock tower brace, Front Bumper Lip Spoiler, JVC DVD Head unit w/ back-up camera, satellite Radio and USB input, 2 - 12" sub-woofers, 4 - 6"x8" speakers, 2 - 6.5" speakers to replace the door subs, 3 amplifiers, LED lights under the hood and in the trunk, chrome e-handle on parking brake handle, chrome cup holder trim, and trunk lid cover with running pony emblem.