Backblaze Backup External Drives after Clean Build


I currently use back blaze to backup my Mac mini with 250GB SSD along with 4 and 5TB USB drives.

I am looking to do a complete rebuild of the Mac mini as it has a lot of apps and solutions I dont use anymore.

I was wondering how backblaze will see the external drives after the rebuild. Even though they have the same data will Backblaze look to back them up again as the main system has been rebuilt? Unsure on how it works. Needless to say 9TB took some time to upload.

Having just gone through this, I can’t really say. I’m inclined to believe that it’s smart enough to not re-upload everything. But Backblaze doesn’t really provide insight in to the difference between rebuilding an index and re-uploading. So all I can say is it feels like it took less time than the original upload for everything on backblaze and locally to be in sync. (Wasn’t checking frequently, but it took ~2 days to reach no new files to upload on the fresh computer and I vaguely remember the original upload taking at least double that time ~700GB of data)

When I migrated from a Mac Pro to a Mac Mini, the backup could not be migrated even though the attached drives’ data was basically the same. I think I remember in a discussion with Backblaze’s tech support that the back up (20+ TB) was simply too large.

I don’t think Backblaze will try to re-upload them from scratch, especially if you use the “Inherit Backup State” procedure, which I believe is specifically intended for when you transfer to a “new” machine. I believe they use small external drive identification files to cross reference and keep track of which drives are new and which are existing separately from your internal drive. I’ve noticed sometimes when something changes in my drive setup it can take a day or two for everything on their end to sort itself out, but it seems pretty smart about figuring out what is existing data that doesn’t need to be uploaded again.


I’ve been through this several times. It’s important to use Inherit Backup State, otherwise it will be treated as a new backup. Data backed up to external drives should be included in this “Backup State” as well. Just make sure these drives selected for backup after reinstalling Backblaze.

You’ll find more information on Backblaze’s support site. I recommend giving this a careful read before moving forward.

On a side note, when you first choose Inherit Backup State it will look like the backup is going to take a long time to complete (months even). If the data has already been backed up, this number comes down pretty quickly.

I also recommend taking a close look at the Backblaze Settings, making sure that there aren’t any unwanted exclusions in place. For example, I believe that disk images are excluded from Backblaze backups by default, even though they may contain very important data.