I would recommend you reinstall 10.10 on to the USB drive using following process.
First restore your system with the old HD and OS to be able to do this:
Connect to your Mac a properly formatted 8GB (or larger) drive, and rename the drive Untitled.
(The Terminal command used here assumes the drive is named Untitled.)
Also, make sure the Yosemite installer, called Install OS X Yosemite.app, is in its default location in your main Applications folder (/Applications). This means that if you moved it before installing Yosemite, you need to move it back before making your installer disk.
Select the text of this Terminal command and copy it:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
Warning: This step will erase the destination drive or partition, so make sure that it doesn’t contain any valuable data. Paste the copied command into Terminal and press Return.
Type your admin-level account password when prompted, and then press Return.
Wait until you see the text Copy Complete, which could take as long as 20 or 30 minutes, depending on how fast your Mac can copy data to your destination drive.