Dual platforms

by 25476 - 1/22/12 7:56 AM

I have my laptop so that PC can be operated as well. The first time I saved the computer to a Western Digital external hard drive, the computer saved the Mac side to the PC side, first erasing all of the PC materials (as well as its platform). The external drive clearly shows on the computer when plugged in. All I remember is plugging it in; WD began backing up when it reached its set time for backups. Question: Is that something I did, or something Time Machine chose to do?