I would suggest that you make sure you have the latest Line 6 MAC OSX drivers installed, they can be downloaded from the link below:

MAC Drivers : 7.1.3

http://line6.com/software/readeula.html?rid=4321

Once the drivers are installed you will need to install the Line 6 Monkey application from the links below:

MAC Line 6 Monkey 1.51:

http://line6.com/getrelease?rid=4619

Once Line 6 Monkey is installed, remove any additional unnecessary USB and firmware devices from your computer when troubleshooting this issue. Make sure to connect it directly to a USB port and not a USB hub, extension, or keyboard of any kind, preferably connect it to a USB port on the rear of computer. Try using additional USB cables and every USB port available until you are able to launch Line 6 Monkey and detect the device.

After Launching Line 6 Monkey apply any updates that are available until you have a green check next to each component.

Next close all open applications and use the License Manager software to de-authorize and re-authorize your device. Make sure to remove the information when prompted to, and then install again when prompted to, this will ensure that you reinstall all of your available Line 6 assets from your account into your device. If you receive the "Failed write to firmware", close license manager and connect the UX2 to a different USB port, reopen license manage and try and to reauthorize the UX2.

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  In the event your issue has not been resolved, we have generated a support ticket for you so we can continue to communicate. Should you need further assistance, please respond to the ticket. With us talking via the ticket, I will mark this thread answered.

  Q: What is a support ticket?

  A: A support ticket allows you to directly correspond with a Line 6 Support Representative. An email will be sent to you from Line 6 containing a link to the ticket when a Line 6 Support Representative responds to it. Please follow the link in the email to view your open ticket to work one on one with Line 6 Support. Also, in your account, click My Account, then click Support Tickets to view the support ticket. Here’s a shortcut:

   http://line6.com/account/tickets/view/

  Please note: Sometimes support ticket notifications will be filtered to spam and junk folders. If you did not receive an email from us indicating that a ticket has been created, please make sure to check those folders in your email host, and also that your email account is receiving messages at all. You may also consider adding Line 6 to your safe senders list.