For years I have backed up my system with Acronis TI to an internal hard drive with no problems.
Everything I read suggests that it is safer to back up to an external drive so I have mounted a SATA drive in a caddy and now use that BUT why is it any safer? It is still connected to the PC via a SATA lead but has its own power source. Should I leave it switched off all the time I don't need it?
Being a belt and braces person after a big problem I now have the following: XP Pro. Main HD - 320 SATA partitioned 60gb for system, programmes. 260 gig for data. I also use ATI 11. I have a matching 320 SATA drive mounted in a removable caddy that is mostly switched off. About once a month (after a spyware and virus scan and a defrag, I clone the main drive to this second drive. I also have an external 500 gig HD. About once a week I do an ATI backup of each of the two partitions on my main drive. Also, I do a copy of "MY Documents" onto this drive, deleting the previous one each time, as I find it easier to keep photos and music this way. I even have a second external HD as a second copy!!
If your power supply goes bottoms up, and takes your drives with it (it can and does happen) - then your backup is still there.
If a software glitch/virus/etc decides to wreck your data, then your backup is still there.
If the bogey man breaks in and steals your computer, then your backup is still there (as long as you keep it hidden or off site).
If your office burns to the ground, then your backup is still there (as long as you keep it off-site)
As you can see, some of these factors are more important in business situations rather than just home use. For many years all the business I do (not so much now I'm retired, but it once was plenty) I do from home. When I went away I hid one backup at home, and gave another to a friend some way away. Thus, peace of mind whatever happens when I'm away.