Today, external consumer USB3 and/or eSATA drives can be a great low cost alternative to tape. For most small outfits, they fulfil the speed and capacity requirements for nightly backups. I use the same rotation scheme with these drives as I did tape with great success.
Unfortunately these drives can't easily be utilised by those running virtualised servers on top of ESXi. VMware offers SCSI pass-through as a supported option, however the tape drives and media are quite expensive by comparison.
VMware offered a glimpse of hope with their USB pass-through introduced in ESXi 4.1, but it proved to have extremely poor throughput (~7MB/sec) so can realistically only shift a couple of hundred GB at most per night.
I have trialled some USB over IP devices; the best of these can lift the throughput from ~7MB/sec to ~25MB/sec, but the drivers can be problematic and are often only available for Windows platforms.
This got me thinking about presenting a USB3 controller via ESXi's VMDirectPath I/O feature.
VMDirectPath I/O requires a CPU and motherboard capable of Intel Virtualization Technology for Directed I/O (VT-d) or AMD IP Virtualization Technology (IOMMU). It also requires that your target VM is at a hardware level of 7 or greater. A full list of requirements can be found at http://kb.vmware.com/kb/1010789.
I tested pass-through on a card with the NEC/Renesas uPD720200A chipset (Lindy part # 51122) running firmware 4015. The test VM runs Windows Server 2003R2 with the Renesas 126.96.36.199 driver. I had to configure the VM with pciPassthru0.msiEnabled = "FALSE" as per http://www.vmware.com/pdf/vsp_4_vmdirectpath_host.pdf or the device would show up with a yellow bang in Device Manager and would not function.
The final result - over 80MB/sec throughput (both read and write) from a Seagate 2.5" USB3 drive!