An external drive should be faster and more reliable than tape.
Have you considered have a service come in over the net and do your save.
Then you will always have an off site copy, archives.
Well, Its not like whether its better to backed up into tape or storage, Its a point to reanalysing/restoring the backup time to time in any backup/additional server.
What is the problem you are having with tape backups? Are you sure that problem will not impact disk backups? Rather than disk, I would prefer an online service.