The placement has little at all to do with viruses. Both external and internal disk drives have the same chance of getting viruses.
Hard disk drives
MSDOS and Microsoft Windows identifies drives by single letters (many other operating system allow drives to be named with words or phrases):drive A the first internal floppy diskette drive (usually not used anymore)drive B the second internal floppy diskette drive (usually not used anymore)drive C the first internal hard disk drivedrives D through Z additional drives, may be internal or external, hard disk drives or optical drives, etc.
Modern disk drive construction more or less requires drives to have at least one partition. Whether the drive is internal or external is immaterial.
for my own opinion it depends on your needs but having external hard drive is more of peace of mind for you if your PC accidentally got virus you got back up files
It is perfectly safe as it will use your internal memory (HDD) to emulate a Disk Drive(s) in order to run the CD(DVD)-image file from your hard drive.
DVD-ROMCD-ROMExternal hard diskfloppy diskpen drive
Usually the average desktop computer the home user would use is one internal storage device (whether it's a hard disk or a solid-state disk).
Disk storage devices are most commonly used in personal computers as a means to enhance memory. The most common are hard disk drives, optical disk drives, and even zip drives.
who is the largest manufacturer of memory is made up of hard disk drives
disk controllor for puzzel chap. 3
A Disk controller.
internal