iMac won't boot off of Mavericks USB when downgrading from Yosemite I made the biggest mistake of my adult life and installed Yosemite without backing up my HDD. Fortunately, I do have most of my actual files and programs saved to my external HDD, but that leads to another problem.

I use a lot of 3rd party apps to make music, and they are not compatible with 10.10 i.e. Pro Tools etc. I formatted my HDD in an attempt to start over but it keeps reinstalling Yosemite from the Internet.

I've made several USB boot drives but when they go to load up it just gives me "international no" sign (see image below).

Can someone please help me as my work/life is tied in to my studio. Oh also my external HDD is now all of a sudden read-only NTFS instead of Mac read/write enabled.
The Booting from USB issue

Ok, so first off. I don't know how you made the bootable USB, but for the sake of consistency in my answer I will give you a method to use: 


Download Mavericks for the App store. (I checked, it is still there for downloading)
Plug in the USB drive you want to boot from (has to be 8GB or more)
Then from the Terminal app run this command: 

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app



It takes a while to complete so go make yourself some coffee and let Mac OS do it's thing. 

NB Notes:


Replace MyVolume with the name of your USB drive. 
If you have already downloaded Mavericks and backed it up somewhere, you can just copy it back into the Applications folder. It has to be called Install OS X Mavericks.app.


Taken from Creating a bootable OS X installer in OS X Mavericks.

If you boot your Mac, immediately hold down the ALT key and select the USB as the boot device. This should boot into the Mavericks installer. From there you can run DiskUtilities and partition you OS hard disk and install Mavericks. 



The NTFS issue

It sounds like you might have had a 3rd party app like Paragon installed. Mac does not natively allow writing to NTFS.