Purposely I avoid to use all of these types of devices while traveling. There are two reasons:

1. They are all based on an internal hard-disk drive which is fragile. One drop and a traditional hard-disk is dead. Having moving parts is what makes it more fragile. In several of models you can get around this by replacing the disk with an SSD which solves this problem for a hefty price but not the next point.
2. These devices are valuable. Because they cost money, the will be a target for thieves just like a laptop or your camera.

Instead I use an Addonics MFR which is a flash-card (Compact Flash, SD, SM, MS, xD) to DVD burner. The device is both AC and DC powered (10 DVDs burned on a single charge) and cost me $330 USD. It has plenty of advantages:

• Storage is cheap because blank DVDs are cheap. You can burn as many as you want.
• Replication is simple. Just burn everything twice or more.
• Distribution is easy. I keep one copy of my photos with me and mail myself a second.
• Disks have no value to anyone else since are already used and were cheap to start with.

The only two downsides I can think of is that your incremental storage is limited to 4.5 GB which is sufficient for me but high-volume shooters may find it limiting. SDXC cards are not supported. You can burn larger CF cards and the MFR will prompt for the insertion of multiple disks (Disk Spanning).

Addendum [EDIT]:

• Portable hard drive with small form-factors have one more issue which is that they stop working above 10,000' (3000m) from sea level. I once traveled with a group of photographers and every single iPod in the group failed once we crossed 3200m of altitude.
• Optical disks are not only virtually indestructible they also have yet to be lost in the mail. I suppose because they have no value. I send myself regularly 4-8 DVDs per trip across continents and they all made it home for a moderate price.