Your answer may be 2 drives. One for Mac and the other for the PC.

Let me state that most external drives fail the basic tests for backup on these 2 points.

1. Write protect switch. Most drives do not have a switch that turns the drive into a read only device.

2. The second copy (and third) copy of the backup. Since most will have just one drive and will be writing to it there is no assurance that the virus or worm won't wipe it all out.

Bob