Hello, as from the thread title, i just plugged in my new my book external HDD and i was asked if i would use it for time machine backups. I clicked yes and TM backed up 30GB. How does it work later? Each time it does a backup it writes 30gb over and over again or just updates the previous backup? Also, if this disk is used by TM, can i also put other files that are stored on my old external HDD or this HD cna be used by TM only? THank you.