MacBook Air M5 and Samsung T7: Time Machine fails immediately, makes T7 read-only

I have a MacBook Air M5 and a Samsung T7 drive. Both are new.

I used Disk Utility to format the T7 with APFS and GUID.

I ran a BlackMagic speed/stress read/write test with 5GB. ~800 MBps read/write. No errors.

But when I try doing a Time Machine back up, Time Machine fails after a few seconds and changes the T7 to read only (!).

Same results with a second T7, different USB-C cables (both original Samsung cables), both ports on the MacBook.

The MacBook is plugged into the wall.

All Samsung software is uninstalled.

I turn off Spotlight for the drive.

I reformatted the drive as Mac Extended and GUID and let Time Machine handle reformatting as APFS. Same results.

The MacBook uses File Vault by default. The Time Machine backup is set to not use encryption and gives a very quick message about that when the backup starts.

Posted on Mar 25, 2026 1:28 PM

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Joe Sixpack Author

Level 2

184 points

Posted on Mar 26, 2026 8:15 PM

Error message was:

"Backup not completed. Time Machine couldn't back up to CleanSlate"

("CleanSlate" is the drive name.)

T7 drive permissions:

Prior to attempting a back up: "You have custom access"
After back up failure: "You can only read"

I'd run EtreCheck, but this is not my Mac, so I don't want to install software that is not from the App Store. I did try watching the console, but it was flooded with messages, and I didn't know what to look for.

I did two more tests:

1) Set the backup to be encrypted. That made no difference.

2) Started backing up from a 2022 M2 MacBook Air to the T7 using the same cable. No problems.

My next step is check for a Samsung firmware update. After that it's off to the Apple geniuses.

-----

MacBook Air M5, 1TB

MacOS 26.4

T7 2TB

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16 replies

Joe Sixpack Author

Level 2

184 points

Mar 25, 2026 9:01 PM in response to HWTech

I thought read-only meant read-only.

Regardless, the question remains: why is Time Machine failing?

MacBook Air M5 and Samsung T7: Time Machine fails immediately, makes T7 read-only