You really need to give us more information. You are “running the roms off a usb” what? External HDD? Flash drive? How is it attached? Directly? Via a USB hub? Does it have it’s own dedicated power?
With the limited info, I’m guessing you have an external USB hard drive plugged in to your Pi and it does not have dedicated power. If so, this is going to be a problem for the Pi – it can’t provide enough power through the USB ports to run an external USB hard drive. You need a good quality USB hub in the middle that powers the drive. Or a drive that has it’s own power supply and doesn’t pull power off USB.