External USB disks
We had this discussion last year, in the context of the cost of a petabyte storage system. This is a slight update to adjust the costs (cheaper disks) and to compare with tape.
An LTO-6 stores 2.5 TB (raw) and costs about $50, or $20/TB. An 4TB external HD, USB 3.0, costs about $150, or $37/TB. The bytes per cubic centimeter are about the same, and the HD cost continues to drop. The potential for compression is much better for disk than for tape, but I choose to ignore this because any compression scheme add complexity that may prevent the data from being recoverable 20 years from now.
I can build an archival storage system with 8 computers each supporting 32 of these drives, with switchable power for each drive. The total cost for the non-disk portion of this system is about $4000, so the system-level cost per petabyte is about $41,000.
This is basically a stack of 256 disk drives that are almost all powered off almost all of the time. Any given file can be accessed by turning the drive on and waiting for it to spin up, so access is about 5 seconds. In a backup/recovery system, you treat each drive more or less like an LTO, so you power up one drive each day and write to it for an hour or so (assuming you have 4TB/day to back up.) Just as with tape, you may choose to back up to two drives at twice the overall cost.
Disk lifetime is driven primarily by the amount of time the disk is powered up, so data retention in this system should be very long.