This is almost the same question as: Mirror or Backup?
When you mirror data, and somthing happens to your controller in PC or the controller in the HD itself, you will mirror the bad data to the mirror drive. Mirro of data does not protect from this fault. You still need backup.
So whats backup? You want to backup maybe once a day, that when something happens to your primary drive, you have time to discover the problem and then restore from your backup drive. When you backup to often, you will most of the time have no chance to discover you have bad data, so you will not restore and you will overwite your good data with the next backup.
This means more often then once a day does not really help, but backup only once a day is also not good enough, because in a lot of cases you will find only out ,that something bad was allready backed up a lot longer after one day. For this reason you need a backup plan, for example you backup once a day (and overwrite the backup from previous day) but you keep a copy every week and also every month and also once a year, so you will have always 4 copies with diferent age to avoid a lot of this issues, but it will never be perfect (ask me, lost almost my data, because one HD was shorting the IDE bus and the backup HD was on same IDE bus....as i know now, never put the backup disk on the same IDE bus!!!). You can combine this that you make every day only a incremental backup and only once a week and month a full backup, books are written about this. You must make a plan for yourself.
So keep in mind, a RAID-Mirror is not to replace a backup.
there is a lot of S/W for this, a good free one I use is:
http://www.2brightsparks.com/downloads.html#freeware
//Wolfgang