I would expect time machine to identify the volume based on its UUID, and be able to tell that it's the same volume, but I will take a look at it and see if it can tell the difference.
If it does indeed see it as a unique structure, it should show up numerous times in my /Volumes folder when I browse time machine, once for each copy it has made. If it does, I'd call it a whopper of a TM bug. But I'll take a look and report back.
It's not a question of Time Machine mis-identifying the drive. Let's give an example. Suppose you have told TM to back up your boot disk which for some bizarre reason you've left with the default name of "Macintosh HD". You also have a flash drive containing a single HFS+ volume that you've cleverly named "Flash Drive". TM is backing up your computer to a folder which might be named /Volumes/"Time Machine"/Backups.backupdb/"My Computer", but we'll just call it "Backups" for short.
At time Time_A, TM takes a snapshot and sees that both "Macintosh HD" and "Flash Drive" are present. It backs them up to:
So far so good.
Then you detach the flash drive, and at time Time_B TM takes another snapshot. This time it sees only "Macintosh HD", so it updates the backup to:
Notice that TM has dutifully record the
absence at Time_B of the flash drive.
Now you re-mount the flash drive, and at time Time_C TM backs up again. You now have:
Whether TM recognizes that the "Flash Drive" that was present at time Time_A is the same drive as the one that was present at time Time_C is irrelevant. What it does recognize is that every single one of its files that was present at time Time_C was absent at time Time_B, and must therefore be treated as a brand new file. It's copying rule is simple: it copies everything that has changed since the last good backup, and the last good backup was at time Time_B.
It would be nice if Apple amended the rule to say: copy everything that has changed since the last good backup
of this source volume, but they haven't done that. (Yet.) This is not mere carelessness. There are technical problems to be overcome, having to do with the extended attributes that TM attaches to the file. It doesn't expect gaps in the timeline for any particular file.
As for looking in /Volumes while in the Time Machine view, that won't show you anything useful, because:
- When in the TM view, TM is showing you what was present at a particular time. Even if it mounted "Flash Drive" as different mount points, it would be mounting them only one of them at a time, and at whatever time you looked the most you could hope to see would be the single mount that was in effect at that time.
- /Volumes is invisible
- Even if you make it visible, it's empty in the backup. TM backs up the folder itself, but nothing inside.
- When in TM view, you can see a list of mounted volumes in the sidebar or wherever you normally see mounted volumes, but TM isn't getting that list from ./Volumes. At Time_A, for example, it sees that Backups/Time_A contains two items: "Flash Drive" and "Macintosh HD". Those are the currently mounted disks as of that snapshot.
