The problem appears to extend to the cloud backups as well if the initial backup set was created using a locally connected external drive. iDrive offers this for an initial setup. I sent a hard drive back after performing a local 700GB backup, and when the files appeared available online, started a scheduled nightly backup. Regrettably, the iDrive app can only perform full backups which, over my DSL line, will take several months. This is a curious deficiency for a product that appears to be relatively 'mature'.
I asked support about this and they appear to be unfamiliar with the problem - yet i followed their recommended route when I signed up for the service (default installation settings, default user folders in backup set, accepted offer of external hard drive for initial backup, etc.). They suggested I upgrade to the latest version of the iDrive app, however the latest version suffers from this same shortcoming: It doesn't do incremental backups against a full backup made to an external drive.
I have not heard back from tech support. I hope I do. I had high hopes for this backup solution.