Apple’s Time machine may be experiencing what appears to be a malfunction. It seems that the backup software is causing some internal drives to be added to Time Machine’s exclusion list which causes some drives to have it's data excluded from back up on the hard drive. The omission of the drives is not reported by the system leaving users unaware of the problem. The issue is most prevalent where multiple internal drives are being managed.
The malfunction can be detected by mounting a drive and adding it to the Time Machine exclusion list and reboot your computer. When the system boots, reopen the Time machine exclusion list and dismount the previously drive.
Those experiencing the problem will observe the drive disappearing from the exclusion list but see it being replaced by another mounted drive that is then replaced by another mounted drive that is then excluded in backups. The new drive’s data will not be backed up. Time Machine’s apparent malfunction may exist in complicated drive setups that have heavily partitioned multiple internal drives. Typically, the Time Machine is challenged to work with one or two internal drives. Time Machine seems to function well with this work load. The more multiple drives in the set up tend to more likely fall prey to the malfunction.
Additionally, the malfunction appears to be more prone to specific setups rather than the cause of a system-wide malady. Yet identifying the problem is problematic. One issue may be in how the Time Machine identifies drives that it chooses to include or exclude. Time Machine excludes is driven by file path. Yet, when managing volumes through UUID’, the problem seems to appear more frequently. Another possibility is that Time Machine is experiencing corrupted files in the Time Machine preferences files where the volume configuration information resides. To avoid this problem, make sure your files are being properly managed as the system expands with usage.