Time machine is stuck preparing backup

Please read If Time Machine remains in the “preparing” stage on Mac.

What about using the cloud as my alternate backup storage device?

Each word of that phrase becomes progressively uncomfortable with its implications:

• "Alternate" is a good idea, but "additional" is a better one. iCloud can definitely be an additional tool in your backup strategy.
• "Backup" is an obvious necessity. If you should lose a file stored locally, having it stored elsewhere is advantageous. But plenty of people inadvertently delete a document stored in iCloud and only in iCloud. What then? Good question.
• "Device" is obvious, but it's a storage device that is not yours. In reality it's many devices, but they're all elsewhere. What do you do should those devices should become unavailable for any number of reasons? Another good question.

Yes use iCloud. Just don't rely upon it exclusively as a backup strategy.

That would have the advantage of one backup on site, one off site.

An indisputable advantage. How about keeping one of your portable, inexpensive, easily replaced, disposable USB hard disk drives off site—a question that logically leads you to the conclusion that you really need a minimum of three independent, redundant backups, with at least one of them kept geographically distant from the others at all times. Sheep, wolf, hay.

On the one hand, I am not inclined to trust the cloud.

My trust in any Earthly thing is limited as well.

On the other, Time Machine backups are encrypted, aren't they? How secure is that?

Very. An encrypted TM backup is completely useless without its encryption password. Don't lose that password. It's also advantageous because when a backup drive fails (not if but when) all you need to do is throw it out. On the other hand if that drive should come into someone else's hands, an unencrypted backup can be used by anyone to restore a Mac that is not yours, in effect making it a duplicate of yours. Solution: encrypt.

I guess there is also the question of speed. If it takers a few hours to do a full backup via my USB port, it might take days to do a full backup to the cloud.

It takes longer for obvious reasons, but no matter whose backup service you use there really is no such thing as a "full backup to the cloud". Apple is careful not to refer to it in that manner, but if you look at other (usually, highly advertised) "cloud" backup services reading their fine print will also lead you to that same conclusion.

There is no substitute for Time Machine. There are plenty of backup strategies but nothing else is as integrated with macOS or is capable of conveying its features.

Oh, a half hour is nothing. If you recently applied a macOS update or upgrade, it might stay in the "preparing" stage for several hours. I know not seeing any apparent progress can be frustrating, but I wouldn't even begin to think about it until tomorrow.

Until then you might consider directing those thoughts to obtaining at least one more, redundant TM backup disk. If you can find no obvious justification for a substantial delay, impending disk failure might be a reason. One and only one backup isn't sufficient anyway.

Nope. Using Time Machine means you don't have to worry about anything.

🙂

Just let it work. But yes, get another backup drive at your earliest convenience. Fifty or sixty bucks. Having two means neither one will ever fail. Having only one tempts fate.

Each backup drive will contain complete, fully restorable system backups, as many as will fit.

TM will sequentially back up to as many backup drives as you want to provide. If one doesn't happen to be available when it wants to back up, it looks for the next one, and so on.

Is TM referencing only the previous data on the current drive.

I'm not quite sure I understand. When selecting a file or folder to restore (because it was erased or altered in a manner you want to undo, for example) TM will search all available TM backup disks for backups of it. You can choose to restore one version of that item, or multiple versions, and keep the original, or replace it.

Can I clone the TM b/up to a new drive (a new SSD to be swapped for the existing system HD)?

You can copy a backup drive to another backup drive using Disk Utility's Restore function. It results in an exact copy. That's often desired when you want to replace a TM backup drive with a larger capacity one for example. But if you're asking if you can swap a startup disk with a TM backup disk the answer is no; that's not how it works. If you need to replace a system startup disk with a new one, you would restore it from its TM backup.

I've discovered when switching b/up drives, that TM can alternate between drives. Does this create 2 separate continuous b/ups A and B or are they split, meaning half the data on each drive?

following on from my previous post....Is TM referencing only the previous data on the current drive. I hope that makes sense.)

Thanks, that is great to know - thinking about it wouldn't work very well otherwise.

Can I clone the TM b/up to a new drive (a new SSD to be swapped for the existing system HD)?

Thank. Good to know.