I can't say for sure what's wrong, but I think that there was (is) data still on the drive even after you "erased" it. I believe you may have erased only one APFS
volume, but not the entire drive (hardware device) itself. This can easily happen because by default Disk Utility shows only volumes, and not the devices (or containers!). So, there could have been more than one volume in the APFS container -- perhaps one that's hidden from the desktop so it would be easy to overlook.
Or, perhaps the Sandisk drive has more than one partition, either accidentally or purposely? (If it's purposeful, you'll need to store any data you want to keep in another location!)
In any case, the remedy should be the same -- repartitioning and reformatting the drive, being sure to select the
top-level device before clicking Erase:
- First, open Disk Utility, go to the View menu and make sure Show All Devices is checked! (By default it's not.)
- Now in the left pane, select the device that corresponds to you Sandisk external drive. It should be under "External" and it will probably have "sandisk" in its name. There will be a few lines "indented" under that Sandisk device, representing the APFS container and volume. (In any case, DON'T pick the APPLE SSD line -- that's your internal storage device!)
- Having selected the proper line, now click Erase.
- In the window, make sure the Scheme is GUID Partition Map. I don't think it matters if you format it APFS or MacOS extended, because Time Machine will re-format it at APFS anyway before using it.
This proceedure will result in the drive having a single partition, with a single volume, if you selected macOS Extended, or a single partition, single container, single volume if you selected APFS format.
Not sure what exactly is going on, but a 500GB external drive is not sufficient for a Mac with a 500GB drive. Apple has recommended in the past that a Time Machine drive should be at least 2.5 times bigger. So would want a 2TB drive at minimum.
I haven't tried it, but I think TM could successfully back up to an external drive that's equal to the boot storage. At the least, the
first backup should succeed. If the boot drive
really only has 400 GB in use, there should be enough room for a few older snapshots. Also, by default, quite a lot of data is excluded by TM, like cache files, spotlight indexes, trashes, and the System Volume itself (about 16 GB). So I think it could work.
Sure, it's not ideal, as not much history would be kept, but I think TM
should be able to handle this situation. As you say, though, perhaps CCC would be a better choice (but not as easy to set up ).