There are a number of utilities that create bootable editions of Linux on a USB flash drive. When running such a utility you may be given a choice. One option is for Linux to logically treat the USB flash drive as a CD, that is, to make it read-only. This is not a hardware thing, you can still modify the flash drive when Linux is not running off it.
The other option is to let Linux save files on the flash drive. The upside is that you can save bookmarks and install bug fixes to the operating system and Firefox, avoiding the need to periodically burn a new CD. The downside is that malware can potentially save itself too. Then again, there isn't much (any?) malware that runs on Linux.
Mint offers the best of both worlds. When run from a USB flash drive, it offers options to run "persistently" or "without saving." For online banking, run without saving. Then periodically, run in persistent mode to install bug fixes.
If you like the idea of booting Linux from a USB flash drive, a 4GB model should be more than sufficient to run any of the popular distributions. When shopping for USB flash drives, be aware that there is a large difference in speed among various USB drives.
For example, OCZ offers three classes or gradations which they call "extreme performance," "performance" and "ultra-portable." Their Diesel and CrossOver models are ultra-portable and the specs say nothing about speed. The ATV and Rally2 models are in the "performance" class but again, the specs omit speed ratings. Only the top of the line Turbo models have specs that lay out the read/write speeds. According to a Tom's Guide review the OCZ Rally 2 Turbo was indeed speedy.
The same is true at Crucial, which offers three different 4GB USB flash drives. The specs for the $19 Gizmo has no speeds. The specs for the $39 Gizmo! Plus does have speed ratings. And the specs for the $58 Lexar JumpDrive Lightning show even faster speeds.
To evaluate other brands, Nirsoft has user contributed speed tests of many 4GB flash drives.
Regardless of the media (CD or USB flash drive), if you run a bootable copy of Linux on a machine with another operating system installed, then Linux may or may not see the files on the internal hard drive. This depends on a number of factors such as the version of Linux and the file system on the hard drive.
You're safer going online with a copy of Linux that sees only its own files, not also those of another operating system. Malicious software or web sites can't read or copy files that the operating system doesn't even see. If your favorite Linux distribution defaults to making the internal hard drive visible, it should be a simple matter to dis-mount the volume (in Linux lingo) that is the internal hard drive.
Finally, be sure to change the password on your router. A router configured to use malicious DNS servers is the only thing that can defeat all this work.
ALSO SEE: The GNU/Linux Desktop: Nine Myths