MIrror a Folder?

There is a "hardware" and "software" alternative:

1. Hardware - create a mirrored RAID using the two drives. Both drives should be identical in size and mfgr., although this isn't essential. The RAID can be created using Disk Utility. In a mirrored RAID everything written to one drive is automatically mirrored on the other drive.

2. Use a backup utility that constantly monitors changes in one drive and syncs the changes to another drive. Synk Pro 7does exactly this. Time Machine does something similar but on an hourly basis and it creates an archive of all changes to any file that hasn't been deleted from the source volume.

Select the folder then CTRL- or RIGHT-click and select Duplicate from the contextual menu. A new folder is made with "copy" appended.

If you want to make a mirrored backup of the folder then try Sync2Folders, Carbon Copy Cloner, Deja Vu, etc. - CNet Downloads or MacUpdate. In fact any backup utility will usually permit folder, file, drive syncing and backup. Note that if you use a TC with TM for backup then you should not make any other separate backups to the same TC.