The main hard drive for my iMac which is named as Macintosh HD is included in the "Exclude these items from backups" list in Time Machine. It's currently grayed out and I cannot remove it, therefore my backups aren't running.
Time Machine is currently disabled for Build 17A291m, which is the build that both developers and the general public are currently using. This is due to a local snapshot bug that does not reflect changes in disk space when files are deleted. Hopefully this will be fixed in the next build.
Too bad for all of us. Apple should not let this happen and leave everyone on High Sierra in a very risky situation, without ability to backup. May be Apple want yo promote third party solution like CCC or Chronosync?
1. It's a beta - an early beta. Why would you need to have the system provide full backups of a version that may have issues, and where Apple may, or may not, provide full information about those issues?
2. It's a beta. Report your issues, and continue to NOT use a beta system, which may or may not be completely stable, for your primary computer.
Yes, everyone on High Sierra is in a (potentially) risky situation. (Not to repeat myself (again), but it is a test system until the final release )
If Time Machine is presently disabled, and you feel the need to keep a full backup, then you will need to use an alternative.
As the previous two posts have alluded to, Time Machine alternatives are a life-saver right now if you need to make a backup of the current High Sierra build. In particular, CCC was an invaluable asset for me this evening in circumventing the storage deletion bug I mentioned in my earlier post.