There was some previous discussion of issues with sparsebundles:
and there’s discussion of issues from a couple of years ago on the Apple forums:
https://discussions.apple.com/thread/252729592?page=3
and I’m wondering if anybody has any knowledge of whether these issues persist in the current OS (and I should thus avoid them!), or whether Apple seems to have gotten its proverbial act together.
Does anybody have more current info?
I’ve been using encrypted sparsebundles on occasion for years and never had a problem. But I just checked mine and none of them can be opened. And neither can copies I just restored from backups made more than two years ago. Not good.
Until recently, I was using encrypted sparse bundles as destinations for SuperDuper (two large spinning drives, each one with 4 sparse bundles, one for each of the machines I back up, two drives rotated off site every 2 week), and they took FOREVER to mount and could only be unmounted by launching Disk Utility. I never lost access to them as WayneG did, but they were a pain. Changed to regular encrypted disk images and have been much happier.
This is seriously bad news. I hope something in the way of an explanation turns up. I had such a bad experience when I tried using self-created encrypted disk images that I stopped using them years ago. But I’ve always assumed that others have continued using them with success. 
So did I, just now.
@WayneG and @EricWEvers, are you talking about “sparse” disk images or pre-allocated disk images? Wondering if the issue is the “sparse” part en toto or if you think it relates to the “bundle” aspect…?
“Sparse bundle disk image:* Same as a sparse disk image (below), but the directory data for the image is stored differently. Uses the .sparsebundle file extension.”
As described in Step 8:
Sorry, lack of clarity on my part. I know that sparsebundle disk images are the problem. I was wondering what you switched to - “sparse disk image” or the pre-allocated style of disk image?
Got it. Pre-allocated space. Just created images the size of the drives I was backing up. Still get a little weirdness with things not wanting to eject all the time, but it’s a LOT better than when I was using sparse images. Hope this helps!
-Eric
I’ve used encrypted dmg files in the past and will probably use them instead of .sparsebundles going forward.
It’s been my habit to backup sensitive files, using ARQ, before storing them in sparsebundles, etc. so I should be able to recover the files in the lost bundles. But most of these are archives that I may never need.
If you want to read more on this topic, The Eclectic Light Company is a great resource. I would start with the earlier article.
Here’s a quote from “A User’s Guide to Disk Images”: Whichever disk image format you use, backing up contents remains essential if you don’t want to lose data. (emphasis is mine).