iMac won't boot off of Mavericks USB when downgrading from Yosemite
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.