Do you have a Microtek FireWire compact flash card reader?
I only have 1 mac here running snow leopard. When I tested it at the Apple store this morning, their MacBook running SL had the same problem: mounts 1st time ok, but after that, forget it
Oho! It finally just dawned on me that I have had the exact same problem... getting cards to mount... either FW or USB. I have 3 readers on my desk right now. The latest I just got.. it's a Kingston and everything mounts ASAP. Other readers... I have to keep trying repeatedly before they will mount. This problem predates Snow Leopard for me though.