Complex backup strategy - SSD system disk AND External USB disk

If you choose to connect your Time Machine backup drive only from time-to-time, try to do so at least every-other day, at minimum. Otherwise, it may take several hours of computation just to decide what needs to be backed up, before any files are transferred.

Time Machine computes the backup list in way less time than expected by using the File System Event Store, a finite-sized record of what folders have recently changed. If you allow the File System Event Store to get stale, Time machine must do a full traversal, which takes hours. Because of the way Time Machine works, it computes its backup list first, before starting to copy any files.

Time Machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.

I think if this were mine, I would get a really big disk and put it all on Time Machine. Then you never face having put off the backups for so long they are really, really old.

Without being able to take advantage of the File System Event Store, all other backups are facing an all-afternoon elapsed time. They will need to examine every file on both the source and destination to decide what needs to be backed up, even if only a few files have changed, reading every file on a big Disk AND a big Backup takes a long time.

¿Are you willing to dedicate all afternoon to JUST doing Backups, and nothing else? How often are you really willing to do this?