Flash drive does not appear in Finder or on desktop

When I plug a flash drive into my iMac, it sometimes does not show up in Finder or on the desktop (or if it shows up when I plug it in, it disappears after awhile). As a result, I am unable to eject it properly, and when I physically disconnect it a message appears telling me I removed it improperly and may have corrupted my data. Another discussion thread said to go into Finder> Preferences and choose to show CDs, DVDs and iPods on the Desktop. I had already done this, so there must be another solution. Any help?