Why won't my mac read my USB flash drive?

Hello there Bast14,

It sounds like you are not able to use your USB ports on your computer. I would use the troubleshooting in the following article to help resolve that, named:

USB and FireWire Quick Assist

http://support.apple.com/kb/HT1151

• Restart your Mac
  Sometimes a simple restart can resolve issues. From the Apple menu, choose Restart to restart your computer. Once your Mac is finished starting up, test your device again.
• Check the software
  See if a newer version of software is available for your device if it requires software. Visit the product manufacturer's website.
• Check your device's power
  Make sure that your device is on if it has a power switch; try turning your device off and then on again.
• Try a different port and cable
  Connect your device to a different USB or FireWire port on your Mac. If you connected your device to a USB or FireWire hub or to a port on another USB or FireWire device that's connected to your Mac (known as daisy chaining), connect your device directly to a port on your computer. Some devices need to draw power from a USB or FireWire port to work; if you connect a device to a non-powered hub (one that isn't plugged in to a power source) or to a low-power port (such as the one on your USB keyboard), it may not work. If you have another known-good cable, try swapping the cable to see if that works. If it does, you should replace the problematic cable.
• Check for device conflicts
  Disconnect all USB or FireWire devices (including hubs and extension cables) except for your Apple keyboard and mouse. Then, connect the device in question directly to your computer. If your device works now, there may be a conflict between that device and one of your other USB or FireWire devices. If it doesn't work, go to tip 8. If you do have a conflict, you can determine which devices are affected. Connect your other devices to your Mac, one by one, making sure to test that each device works before making another connection. When you find the device that causes the issue, contact the manufacturer for assistance (check both manufacturers' websites for software updates and additional information).
• Quit associated applications
  If you're using a particular application that uses the device in question (for example, you're using Image Capture with a USB scanner), try quitting the application and then opening it again.
• Try another user account or computer
  If you have another Mac available, try connecting your device to that other computer to see if it works. If you don't have another computer available, log in to another user account on your Mac (or create a new user account and log in to it), and test the device again. If the device is working now, there may be a software conflict between the device and something in your original user account.
• Check System Profiler for hardware Issues
  Open System Profiler (from the Apple menu, choose About This Mac, then click More Info) and then click either USB or FireWire. If you don't see your device listed in the pane, the issue could be with the device itself or your computer's USB or FireWire port. To rule out the port, plug the device into another USB or FireWire port on your Mac and then check System Profiler again (press Command-R to refresh the window). Also try resetting the FireWire bus. If you see your device listed in the pane but it doesn't work, you may want to contact the device's manufacturer for assistance.

Thank you for using Apple Support Communities.

Cheers,

Sterling

A simple thing like restarting the computer often takes care of a problem. I should have thought about it sooner. It often works. Thanks for the reminder.