Currently on 15 day free trial of Acronis TI Home 2009....running WinXP with 150G internal HD of which currently I'm only using 15G, including operating system. Have a USB external 100G HD for backup purposes and this week I attempted to create a full disk image on this external drive. After clicking on "create an image of full disk or partition", I was give a choice of backing up sector-by-sector, which it said would require more space. Had a grand total of only 15G to back up, so thinking I had plenty of HD space on my 100G drive, I chose sector-by-sector backup. Went thru all the steps and the operation seemed to be working great....WRONG! The backup filled the entire 100G of my external drive, then quit....said I had run out of storage space. So I started the backup again, but didn't choose sector-by-sector backup. All went well and backup was completed, but the external backup shows to be only 8.3G in size. I had 15G to back up, but the backup on ext. HD shows to be only 8.3G in size. Are the non-sector-by-sector backups compressed? Also, would I have to have at least 150G backup space to be able to backup sector-by-sector? What is sector-by-sector backup and is there any advantage using it? Hope I've made my situation clear enough and thanks for any answers to my questions!