How to deal with Carbon Copy Cloner backing up various OS versions of a given machine I've been backing up my MacBook Pro for 5 years using Carbon Copy Cloner. The full backup is on a partition of an external hard drive and there is a folder called _CCC_Archives which I believe stores the diffs over time. 

I have upgraded the OS at least once and I'm wondering what the implications of this are.

Since upgrading from something like Snow Leopard to Sierra some of my music projects fail to open, presumably due to incompatible software. Is there some way to boot my MacBook using a backup from an older OS, capture and export some files, then reboot the MacBook to the newer OS that I'm running?

Note that I have never backed up the Mac since upgrading the OS because I did not want to corrupt my backup if it would in fact be irreversibly corrupted by this change. So the backup drive only includes backups using Snow Leopard.

The setting I use for backups in CCC is called:
"Maintain a backup (Archive modified and deleted files)"

And below this I see:

• Files and folders on "My Passport" that are not present on "Macintosh HD" will be archived.

• Previously backed up files on "My Passport" will be updated only if the size or modification date differs.

• Older versions of updated files will be archived.

• CCC will not prune archive content.
Based on the info in your question you should be able to restore an appropriate backup image to another external drive. 

Once you've done this, boot your Mac into the Startup Disk Manager (i.e. press and hold the option key as you boot up) until you see the drive options appear. Select the disk you've just restored the Snow Leopard backup to and press enter to see if you can boot from it.

If it boots okay, then you can try to convert the files you want to use in macOS Sierra.

NOTES:

1 If your drive doesn't appear in the Startup Disk Manager, there are a number of possible reasons for this. If this happens to you, then while you're in the Startup Disk Manager you can try pressing the option+shift+command+. keys to see if your external drive has optional ROM firmware available.

2 If you can't boot up from the drive, refer to the Help! My clone won't boot! page for a detailed list of possible causes/solutions.