How to resolve Time Machine backup failure after updating to macOS Tahoe 26.2?

On Tahoe there is not a formatting issue but a low-level snapshot or power-management interaction during the first APFS snapshot crawl, which is the heaviest thing Time Machine ever does. On silicon, Tahoe tightened background task throttling and external disk sleep more aggressively, and some SanDisk enclosures firmware mis-handle repeated sleep/wake or UASP resets mid-scan, causing Time Machine to silently stall forever rather than error out.

You can run tmutil startbackup --block --verbose in Terminal with the drive directly attached and watch where it hangs, if it consistently stops after hours with no I/O, disable external disk sleep (pmset -a disksleep 0), reboot, and retry once. Also check log show --predicate 'subsystem == "com.apple.TimeMachine"' --last 1h for repeated “snapshot invalidated” or “device not responding” messages, which confirms this path.

If that still fails, then it is clear that Tahoe + that enclosure combo is broken at the firmware level right now, ChronoSync is fine short-term, but long-term the fix is either a different enclosure or waiting for a macOS point update that relaxes snapshot retries. This isn’t user error, and reformatting won’t touch it.

111alex wrote:

They never finish the inital backup. Tried different drives and formats.

Trying different formats won't work.

• Make sure no third party disk utility tools are in use for the drive. Manufacturers bundle those tools with their drives but they often interfere with normal MacOS operations. If you have any, completely uninstall them and reformat the drive (after making a safe copy of whatever files were on the affected drive).
• Format an external drive as GUID/APFS. Go to Time Machine Settings and remove whatever drives you had there to back up to. Then add your newly formatted drive as a new Time Machine backup destination.
• Connect the newly formatted and empty backup drive DIRECTLY to your Mac (don't use a hub or dock -- you can try using a hub or dock later after your backups are working again).

What you experienced does not normally happen. When I upgraded my MacBook Pro from Sequoia to Tahoe I simply continued to use my existing (two) Time Machine backup drives and everything continued as before.

I use a SanDisk Pro G40 2TB external SSD. I have it partitioned into 2 x 1TB partitions. Using 2 separate partitions with Time Machine on the second partition is working fine. Both partitions were created using APFS.

iamshivam wrote:

... some SanDisk enclosures firmware mis-handle repeated sleep/wake or UASP resets mid-scan, causing Time Machine to silently stall forever rather than error out.

That may be true but it seems to be contradicted by this:

Johne154 wrote:

I use a SanDisk Pro G40 2TB external SSD. I have it partitioned into 2 x 1TB partitions. Using 2 separate partitions with Time Machine on the second partition is working fine. Both partitions were created using APFS.

If it is only "some Sandisk enclosures' firmware" that are incompatible, perhaps some Sandisk drives will work and some will not.

Also noting that on Amazon, the Sandisk Extreme Portable SSD has nearly 90,000 ratings/reviews with an average of 4.6. Including many Mac users. I don't have a Sandisk external SSD but do use external WD SSDs that look like they use the same enclosure. (WD acquired Sandisk several years ago and merged the WD and Sandisk SSD product lines into one unified product line.)

Also, the Apple Store sells Sandisk external SSDs:

SanDisk Extreme® Portable SSD - 1TB - Apple

Failing how? Not starting? Not finishing? Error message?

FWIW, I routinely back up my M4 Pro MBP to an NAS and a pair of 4 TB SSDs, all have worked fine on Tahoe (26.0 on launch day, now on 26.2).