Time Machine now hangs up after latest BigSur update 11.6

I have an M1 mini running macOS 11.6. My 1 TB Crucial SSD that I use for Time Machine backups behaves just like it did when I purchased this mini in Nov/2020. Every hour that the Mac is powered on, without issues. If your Time Machine drive is corrupted, or you haven't done a successful backup with it for some time, the Preparing backup phase will endure longer than you want, especially if it is using a slow, external rotational drive, exceeded available RAM with too many concurrent applications, or in the unneeded company of an anti-virus application.

I suggest that you temporarily shut off Time Machine in System Preferences. Then launch Disk Utility and perform first aid on that Time Machine drive. Green is good, red means run first aid again… or that your drive cannot be fixed. Then switch Time Machine back on and force a Back Up Now.

How is your Time Machine drive formatted?

And, do you have any files on Time Machine that you don't have on your boot drive that you absolutely need?

The reason I'm asking is that with Big Sur Time Machine prefers APFS as the format and that has made it more reliable. So if it's not APFS and you don't have any files you absolutely need on the drive you could erase and formate it as APFS - Case Sensitive with GUID partition map and start over from scratch.

One other point of clarification - my initial post on the failure of TM on my Intel iMac ended up being a disk full error writing to my backup SSD. It seems that TM did not delete my old backups on an encrypted SSD. Once I manually made room on the SSD, the Intel iMac TM worked fine under 11.6.1.

Further to my first post, I have log files for the failure in to Apple Support for analysis. However, working in parallel with Apple I found an odd 'fix' for now. I use Paragon NTFS on both the M1 and the Intel iMac. The latter works fine for TM. In trying to see what might affect my M1 differently than Apple's lab machine, I looked at the Paragon NTFS version and found that it did not update automatically (do not know why), so I manually updated to version 15.9.314 (latest). I tried TM again both with NTFS disabled and with the updated drivers. Neither worked initially. However, I went into Safe Mode after updating NTFS (with it enabled) and TM worked in Safe Model (in my initial call to Apple Support a week ago they tried a Safe Mode TM run, and it failed). I then rebooted to normal operation and ran a quick TM over the top (not much to change) and that also worked. So if you have system drivers like NTFS or other system software, that may be the first place to look for a fix. Hopefully, Apple will address this more rigorously than this trial and error method. But for now, I have TM working again.

How much free space do you have on your boot drive? If there's not enough space for a new local snapshot Time Machine will not proceed with the backup.

I have same issue on both M1 MBP and Intel iMac once updating to 11.6.1. Time machine goes to end then freezes (for 30 min or more) on 'waiting to complete first backup' - this after I wiped off the USB SSD after the first try at backing up that said files were locked and it could not continue with backup until files were unlocked (nothing was locked). This has happened on two disks and both machines, so it is not the disks. Disks are APFS format. Both worked fine for TimeMachine under 11.6 before update to 11.6.1.

The boot drive is where your system and user files are located. If you haven't renamed it it is named Macintosh HD - Data,

Another victim of 11.6.1 breaking time machine. It fails both on my Synology NAS (that has worked for months), and on a freshly formatted external hard drive. Its a MB Air 2020 M1. Time Machine gets stuck waiting on first backup, and never shows a completed backup. The sparse bundle file on the NAS is 95 gb, so something was copied.