Not in the usual sense of plugging a standard external hard drive directly into the T3i/600D. For computer-free backup, the practical solution is a standalone photo storage/backup device with its own memory-card slot. These devices let you insert the SD card and copy the files to internal storage with a few button presses, without needing the camera or a computer. Similar standalone units have been sold by brands such as HyperDrive, Wolverine, and Epson. So yes, you can back up the photos without a computer—but typically by using a dedicated card-to-drive backup device rather than connecting a normal external HDD straight to the camera.