Recently, my MacBook's Time Machine backup to a network disk (on a Mac mini) gives following error:
The exact error message was:
Time Machine detected that your backups on “destination” can not be reliably restored. Time Machine must erase your existing backup history and start a new backup to correct this.
This happens after a kernel panic.
After some digging, it does not necessarily mean the backup disk image is corrupted, but in some cases, Time Machine will 'mark' a backup disk image as 'bad' and refuse further backups to it.
To bypass this issue without erasing the backup history, locate the backup disk image (sparsebundle) in the network share. The disk image file might be in a "locked" state. Use ⌘ + I to open the info panel and unlock it.
Then right-click → Show Package Contents, where the token file might also be locked. Unlock it similarly.
Next, use Xcode to open file com.apple.TimeMachine.MachineID.plist. Change the value of the key VerificationState from 2 to 1 and save the file.
Now we should be able to complete backups, but make sure to verify the backup after the first completed backup. If the verification succeeds, then the backup disk image is not corrupted.
Translated from my Chinese blog post: Mac 时间机器“检测到备份无法可靠恢复”的解决方法