The 90MB message can mean a corrupt file. Could be firmware, could be your music. It would help if you had a Windows computer available to format from the PC to get rid of all your user-added files, but if not…
You can still update firmware from the Mac, and it’s worth a try. Go to the firmware update sticky and instead of the updater link, get the All Regions link for your hardware, version 1. x or 2.x. It’s a .zip file. Unzip it and connect the Fuze and drag fuzeA.bin onto the driveletter of the Fuze–if it’s D: FUZE then drag it onto D:. When you disconnect, the firmware should update.
(If you don’t know what your hardware is, connect to the computer and look for version.sdk. Open that with a text editor and it should list the version. And if you don’t see version.sdk, disconnect and shut off the unit. Slide the power button down to put it on HOLD–it will show an orange dot–while it’s turned off. Connect the USB cord to the computer, hold down the << side of the wheel and connect the USB to the Fuze. This forces the Fuze into MSC mode, which is the way the Mac can communicate with it.)
See if the firmware update helps. If not, the easiest thing to do would be to find a Windows computer, force MSC mode (as above) and right-click on the Fuze. Go to Properties/Tools/Error-checking and run it to see if it can fix or delete any corrupt files. And if that doesn’t find anything, then right-click on it again and Format… to FAT32.