Unibeast is set up on a Mac, but it can be any Mac boot disk. No Windows involved, promise. I made the mistake of thinking I needed Windows and it made the process 1000 time worse.
Here's what to do.
If your mac has a OS X installer DVD, start up from the installler DVD, and as soon as it's booted up from the disk, go to the menubar and select "Disk Utility." Now that you're booted from an installer disk, that 64G flash card won't be read-only. You can go ahead and format it however you like.
If you don't have a bootable OS X installer DVD, then go to macupdate.com and get "DiskMaker X" - it's a handy little utility for creating OS X bootable thumb drives. Then go into the AppStore and re-download the Mavericks installer.
Once the Mavericks installer is done downloading (it's an app called "Install OS X Mavericks" in your /Applications folder), run DiskMaker X.
When it asks, "Which version?" click the 10.9 button.
Then it'll say it found a copy of the Mavericks installer in /Applications, do you wish to use that copy? Click "Use this copy."
Then it'll ask "Which kind of disk will you use?" You should see a button saying "An 8 GB USB thumb drive" - click that one.
It'll present you with a list of thumb drives plugged into your Mac. Ideally there should only be one to choose. Select it and "Choose this disk."
Next, it'll warn you that you're about to completely wipe the thumb drive clean and overwrite it with a bootable Mavericks installer. Click "Erase then create the disk."
Finally, it'll let you know you're about to need your admin password. Click "Continue."
Now it'll start the process. Your thumb drive will disappear from the desktop, and will be temporarily replaced with a generic disk icon with a gobbledygook name. Other windows will open and a progress indicator (which doesn't actually indicate the progress) will appear. The whole process can take 20 minutes, or more. Remember, it's reformatting the drive then copying over 5GB of data to a thumb drive. Not the fastest read/write speeds...
It may occaisinally pull focus back to DiskMaker X script, and it'll ask for an admin password at some point, but you can use Safari or Mail or some other non-fullscreen app while it's doing all this, if you'd like.
Once it's all said and done, you'll have a bootable thumb drive with a 10.9 Mavericks installer, which is something nice to have, in and of itself. For now, though, all you need to do is to reboot from it, and just like if you had a bootable DVD, once it's running, go to the Utilities menu and select Disk Utility and then you should be able to format that pesky ExFAT 64G flash drive.
