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I have an external hard drive that I've separated into two partitions: one that I use for additional file storage, and one that I use as a dedicated Time Machine backup disk. Something has gone wrong with the Time Machine partition, such that it will no longer mount, cannot be repaired, and cannot be erased on its own, neither by using Disk Utility nor through the command line directly, so I've decided to just erase and reformat the entire drive.

I used Time Machine to backup the file storage partition onto a second external hard drive (by selecting the second external hard drive as my backup disk and excluding all volumes other than the file storage partition of the first external hard drive from the backup), with the plan of then restoring this backup onto the first external hard drive after it's been erased and reformatted.

Having never restored anything using Time Machine before (at least aside from after upgrading to a new version of macOS), I was expecting that the backup would be stored on the second hard drive in some sort of archive file-esque format, with the individual files being inaccessible until the backup has been properly "Restored" somewhere else using Time Machine. However, when I opened Finder after the backup had completed, I saw that the result was a lot more like I had just copied and pasted everything from the file storage partition onto the second external hard drive.

Aside from the fact that everything is stored within a folder called "Backups.backupdb", it really seems like I've effectively just transferred everything from the file storage partition of the first external hard drive onto this second external hard drive; the backup folder on the second drive is approximately the same size (just a few MBs larger) as what was stored on the file storage partition of the first drive, and every file I've tried to open from the second drive (pictures, pdfs, mp3s, etc.) has worked fine so far.

So, my question is, is there any reason I should bother "Restoring" this backup to the first hard drive, or can I just start using the second hard drive the same way I was using the file storage partition of the first hard drive before, and then just use the first hard drive for something else after I erase and reformat it?

The only potential issue I can currently see with doing the latter is that the backup folder on the second drive does seem to be in a sort of "read-only" mode, as I'm prompted to "Authenticate" with my password when I try to transfer a file into one of its subfolders, and maybe this is reason enough on its own to bother going through with "Restoring" the backup to the first drive. However, I don't really know much about the specifics of how Time Machine works, so I'm also interested to know if there are any other issues associated with using a Time Machine backup folder like a regular storage folder that I'm not aware of.

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No - you don't need to have two copies of the files you backed up.

I think it's a good idea to restore all the files you want to have a backup copy of, however. Spotlight search will let you read / search all the files, but you have no safeguard for when the backup disk dies or fails, so your retention policy for everything you don't copy back is until that disk has a problem.

Files within Backups.Backupdb are intended to be read only, so you shouldn't expect to ever make changes there (or even be able to delete them) so if you can modify them - you have a backup that's disabled or malfunctioning and making a restore so you can set up a new backup is highly advisable.

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