USB now read only, can't erase (Mac)

Check the device for errors using your OS. This isn't technically an Ubuntu issue at all. You have a USB device that you want to but cannot format. I suggest you post on a Mac or Hardware forum.

No errors. I've owned the USB for over a year and have never had issues with it until now when trying to erase the Ubuntu files.

You could try booting from the Ubuntu Install CD and erasing it in GParted.

To do this, put an Ubuntu Install CD in the drive, boot the computer, select Try Ubuntu, and when the desktop comes up run Applications > Accessories > GParted Partition Editor. Be careful with this--you could accidentally destroy data on other drives besides the external USB drive (such as the internal drive that I presume you have Mac OS X installed on).

If you need help using GParted, please feel free to post again.

If GParted fails to erase the disk, then the error message it gives might potentially provide insight into the problem (which, in that case, is almost certainly a hardware problem).

If you want further help trying to do it in Mac OS X, however, then as actionparsnip says, you will have much better luck on a Mac OS X forum.

"No errors. I've owned the USB for over a year and have never had issues with it until now when trying to erase the Ubuntu files."

Some hard disks are manufactured with serious defects, but most of the time, defects develop over time. Maybe this is the first sign that something is recently wrong with the drive. Have you scanned the disk for errors?