Pretty much any off-the-shelf external unit is going to suit your needs. WD & Seagate are pretty much all you're going to find at a brick and mortar. For 500GB you're looking at between $65-$85 depending on features.

Now if you want to all but guarantee your disk never fails, I would suggest one of Kingston's HyperX or OCZ's Enyo series external SSD's; (If you can sacrifice some of the disk real estate you originally wanted) as those can be had for around $120 for a 64GB unit. Wouldn't be ideal for video storage, but pics, documents, etc... It should be fine.

1998 Eclipse RS-T - Totaled 4/23/09 -
1998 Eclipse GST - Stroked 2.3l, HX40 fed: Build in progress
2010 Subaru Outback Limited

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