Scaling

1.5Mbps ADSL uplink = 675MB/hour = 16GB/day.

50GB home directory = bit over 3 days continuous to make a full backup. So you don't want to do it too often.

1TB disk, completely = 61 days continuous, really slow. Certainly unlikely to complete in one go. Not great to have the first files be two months old by the time the first pass completes.

Performance is even more important for restore than for backups - need fast random access.