How To Let My MBP know that a USB is bootable?
First of all, unetbootin or any other ubuntu usb flasher wont "break" your usb. If your usb is givving you an error when plugged into mac os saying"unrecognized drive",  then this is GOOD! Mac os cannot read a linux filesystem by default. It definitely cant read an ext4 filesystem. The only linux filesystems mac os can read are ext2 and maybe ext3, but even then, that is with the use of external software like Fuse. The linux filesystem is not native to Mac OS, therefore, the os cannot read the disk. However, the linux filesystem CAN be read by intel hardware (which all apple macs use now). Therefore, your mac will be able to read the flashed ubuntu usb and boot the ubuntu installer by you either holding the alt/option key on boot, or if that doesnt work, by installing refind boot manager and using that as a bootloader to boot your usb. If you are trying to dual boot mac and ubuntu, you will most likely need to download &amp; install refind anyway, as that is how you should be selecting which os you want to use at boot. Lastly, because linux is not recognized by Mac OS, it is doubtful that there is a way to get your ubuntu partition (or usb for that matter) to show up in the system preferences boot profile. Once again, to boot your ubuntu partition you must hold the proper start up keys for disk selection at boot, or install refind.

The only possible way that these methods won't work or your bootable USB won't be bootable, is if you are using the following outdated hardware:


PowerPC based Mac
a much older 64 bit mac with a 32bit efi (such as the 2006 core 2 duo models)
An older mac that is 32bit using a 32bit efi.


It is highly doubftful that you are using of these types of macs as they are almost 20 years old. Therefore, if you are not able to boot your ubuntu install disk, it tells me that you are doing something wrong, as opposed to the USB stick being "Broken".