Virtual store too large to back up
I bought a Drobo 5C for my Photos library, which is over 400GB large and growing steadily. Having moved the library to that external storage, Time Machine now refuses to back it up to my TimeCapsule, which has 1TB of available space. What am I doing wrong? Drobo 5C provides users with friendly and expandable storage, which seems like it would be a good choice for storing an ever–growing library of photos.
When formatted as a single volume using the Mac OS Extended file system (HFS+), the Drobo 5C reports to macOS that it has a total capacity of the largest size that it’s able to accommodate — in this case over 60TB. Time Machine is usually able to back up to a smaller drive than its source, provided the amount of data being backed up fits the destination’s available space. But there appears to be a peculiarity in the way that the Drobo 5C reports its capacity that prevents this; it appears to cause problems with macOS’ ability to assess whether the Drobo can be backed up.
If you want to store your Photos library on the Drobo, the best way to do that is to repartition it using Drobo Dashboard — the official documentation should help you with that. However, as it runs a custom virtual HFS+ file system, doing so will erase its current contents completely, so you’ll need to move the Drobo’s contents to a different drive. Only when that’s done, create a smaller volume of around 1TB on the Drobo, which should better match the capacity of your Time Machine’s storage.
Drobo’s external, multi–disk systems are very flexible, but can cause confusion for Time Machine.