I have heard suggestions here to have an offline computer where keys are stored. Even transactions that need to be paid can be saved, entered into the offline computer, signed, and the confirmation transferred back to the flashdrive to be uploaded to the internet. But this requires a flashdrive to go back and forth. So how much does that compromise security?

If one only transmits specific files back and forth to the flashdrive, is it safe? Or is it possible for a program to install itself unknowingly on the flashdrive while in the internet-connected computer?