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First reset the System Management Controller of your problem Mac. Follow the steps below;
Shut down the computer.
Unplug the power cord and also disconnect all other devices.
Wait 20 seconds seconds.
Attach the power cord.
Wait five seconds, then press the power button to turn on the computer.
Then reset the PRAM and NVRAM of your Mac. Follow the steps below;
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup. Then connect the memory stick to your Mac.
Does it recognize now ? If not, in Finder click on Finder menu and select Preferences. Under General tab, make sure Hardisks, removable disks etc are enabled.
Please let me know if that didn't work, so we can continue troubleshooting.