There are currently only three hard disk manufacturers: Seagate, Western Digital and Toshiba, so no matter what brand name is on the case, one of those three brands of drives is inside. Nearly all, if not all, external drives use consumer-grade disks. This isn't necessarily a bad thing, since they are relatively inexpensive and, according to a study by Google, pretty much as reliable as enterprise-class disks.
Having said all that, external drives fail for many reasons, and all brands fail. I just had a Seagate portable drive fail at one and a half years. In this case the failure was the drive itself, but often it is the interface electronics in the case that fail.
If I were buying a new external drive, I would look at cost, capacity, interface, warranty, and the reputation of the vendor (if it is not one of the three disk manufacturers). Also, if the external unit is not branded by one of the three manufacturers you really don't know which company actually made the disk inside.
By the way, the warranty has nothing to do with the reliability of the drive, but it does allow you to get a replacement or your money back if the drive fails. This means you may want to look at the retailer's policy about warranty returns. I bought my drive from Costco and I was able to return it for a refund. Other retailers require you to deal directly with the manufacturer, which can often be a frustrating and time-consuming process.
I ended up replacing my failed Seagate portable with a Seagate 3 TB desktop unit. Interestingly, the warranty on the old drive was three years and the warranty on the new one is two years. As disk capacities increase the length of warranties seems to be decreasing.