backup fails with external volume mounted

Hi,

I get errors like this:

2024-10-13 11:41:36 Failed to enumerate URLs under /Volumes/Seagate Expansion Drive for SnapshotStorage reuse with error Error Domain=NSCocoaErrorDomain Code=257 "The file “Seagate Expansion Drive” couldn’t be opened because you don’t have permission to view it." UserInfo={NSURL=file:///Volumes/Seagate%20Expansion%20Drive, NSFilePath=/Volumes/Seagate Expansion Drive, NSUnderlyingError=0x600000407b10 {Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"}}

after the command

printf '\e[3J' && log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 6h | grep -F 'eMac' | grep -Fv 'etat' | awk -F']' '{print substr($0,1,19), $NF}'

when I leave an external volume mounted during a regular Time Machine backup.

Without the external disk connected the backup goes smoothly!

I moved the Photos library and some other big folders to the external disk because my new MacBook Air 15" has a smaller SSD than my previous 13".

I did check the external volume with Disk Utility. It was only possible to do so by starting up in Safe Mode, but it did return an okay. It is a new USB 1Tb disk after all. "Unable to unmount volume for repair. : (-69673)" when I don't first go to Safe Mode.

Typically the Time Machine backup operation seems to go well, until the last few minutes. Those last few minutes actually take hours and then Time Machine gives up with the "couldn't complete" error.

I leave all applications turned off (except for background processes, I did not go that far yet).

Please advice how to proceed!

In the user interface it appears like this: