Yes, the “insufficient space” problem (bug, dammit!) is ridiculous, and the fact that it’s been corrected is laudable (but should never have happened in the first place; I wonder how seriously Apple takes quality control nowadays). But an even bigger and related problem is that Apple has made bootable clones impossible (and along with that, making repopulation of the Mac’s drive with the clone impossible, too). Dave Nanian (SD), Mike Bombich (CCC) and others (TTPro) may be able to successfully re-enable this, but there’s no guarantee.
But I put the blame for this squarely on Apple. If Nanian et al succeed, then Apple should have been able to allow it, too, from the get-go. And if they don’t succeed, Apple should themselves correct the situation. In any case, it demands correction, because this capability is necessary to be able to quickly and efficiently recover from any really bad situation which may occur (like this space problem). I’m not sufficiently tech-savvy to know why Apple did it, or felt they had to do it (security?), but I can’t imagine that it absolutely needs to be that way.
Looks like the latest version removed a feature (the ability to see and back up the data volume independently from the system volume) that the workaround depends on. I wonder why he can’t put that feature back, but maybe the code’s organization would make that too difficult to do without unwinding other recent features.
I’m not sure why he’s got this phobia against using ASR to back up the system volume, which everyone seems to agree works fine on Intel Macs even though it is currently broken on M1 Macs.
Here is Carbon Copy Cloner’s approach, which is now clear after Big Sur has gone through some system updates:
If the system volume has not been updated, CCC does an incremental update to the data volume clone, preserving the boot status of the backup. However, if the system volume changes, CCC offers the user a choice of just updating the data volume clone or rebuilding the whole boot volume. Updating only the data volume destroys the ability to boot from the clone while allowing it to be a source for migration after a new system is installed. Rebuilding the whole clone preserves the ability to boot but takes longer.
For example, I create a clone nightly from my iMac 2TB internal SSD to an external USB hard drive. When there is no change to the system, it generally copies 10-15GB of data in 30-40 minutes. However, when there was a system upgrade (from 11.1 to 11.2 and again from 11.2 to 11.2.1), the full run took 2.25 hours to clone 1.1TB of data.
Too bad Apple can’t count. We cleaned the drive so 32GB was free, then this. Finder thinks 31.9GB available. Installer thinks 26.5GB is available. How hard can this be Apple.
This is the .1 installer obviously since it gives the warning. Updating from Catalina. Was waiting for things to settle out since no to us important new features.
Actually, determining exact amounts of free space can be quite difficult these days. For instance, with APFS, copies of files may be able to share storage with the original. Plus, with privacy sandboxing of folders, data in those folders can be hidden such that nothing can figure out how big they are.
Now, that’s not to excuse Apple from creating a system where the Finder and the macOS Installer agree on how much space is free, but just to note it’s not a trivial problem.
I don’t intend to pile on here. Clearly this is an Apple blunder.
Nevertheless, I wanted to point out that it’s always been considered hazardous to run disks so close to full capacity. SSDs have changed that somewhat compared to the old HDDs because of the way they write data and their built-in housekeeping, but also because of how it affects their performance compared to a HDD. That all said, it remains true that running a disk (SSD or HDD) almost completely filled up is a disaster waiting to happen. In the old days, the rule of thumb was never below 10% unused, 20% was better. Even today with SSDs, if you have a 500GB or larger disk, IMHO you should really try freeing up more than just a paltry 32GB.
Another apparent upside to having more space is speed. It was my wife’s MBP and after Big Sur install, now has about 50GB on 512GB drive and she reports that her machine is much more responsive. Could be the usual it’s faster because it’s new; but I’m inclined to believe her impression.
Yes, sufficient free space will let virtual memory swap to disk better, so it will provide better performance. I suspect that operating system developers don’t put a huge amount of effort into improving performance in highly constrained resources scenarios anymore simply because resources like memory are so cheap to expand.
Coming in late on this because I just got bit by the bug on my circa 2018 MacBookAir with 128 gig of storage and it is now crawling its way through a Time Machine backup of the previous Mojave system. I did get a warning that I needed to clear more memory, but I may not have succeeded. I cleaned out iCloud Drive so it shows 14.13 Gbytes on Get Info my macMini, but Storage under “On This Mac” shows 29.8 Gbytes. That’s a rather large inconsistency, and may have tripped up the Big Sur install.
I bought the Air on sale a couple of years ago, and it is essentially a backup machine which I mainly use when I want a computer outside my office. I also use it as a test machine for debugging, and in this case to test if it’s safe to upgrade the MacMini from Mojave to Big Sur. Now that I have established that the short-term answer is NO, I’m wondering where to go from here.
After recovery, On My Mac tells me the Air has only 6 Gig of 121 gig available, with 72 Gig of “Other volumes in Container,” numbers which seem to be fluctuating as the Air sorts through the recovery. Do I need 32 Gig for recovery? How can I tell what the “other volumes” are? I don’t have many non-Apple apps or documents on the machine, and I’m wondering if it’s possible to strip it down enough to upgrade without making it unusable?
Would trying a Catalina upgrade first do any good?
What does it tell me about the prospects for upgrading the Mini to Big Sur? I am a heavy Apple Mail user, and from all that I have read about the problems with the Catalina version of Mail, it sounds that I don’t want to go to Catalina unless I immediately upgrade to Big Sur?
FWIW, I did read Take Control of Big Sur, although it’s entirely possible I missed something.
