Is it good idea to use RAID 1 mirroring to back up database content
Relying on RAID 1 as a backup is not a great idea. If you had data corruption in the database, it would likely appear on both drives - certainly if it were caused by faulty SQL commands being executed. RAID 1 will only protect you from physical failure of one of the two drives, not corruption caused by a duff application or end-users.

In addition, backups that are stored on the same machine is also asking for trouble. You should always store backups off the source machine, and use something like mysqldump or the Percona Xtrabackup tool to perform these backups.

If you can mount an external volume onto the machine through NFS (or CIFS if Windows), then use Xtrabackup to backup the database content onto the additional volume, which is physically on a separate machine, then if you lost the machine, you could rebuild and restore from the backup.

Ensure you also backup your configuration, such as OS users, MySQL configuration files, etc. as without these, restoration may prove trickier.

Hope that all makes sense?

Dave