I don't see an issue with the Seagate 500s you already have, they'll do fine inn a RAID.
When it comes to Enterprise drives the issues are in firmware and instruction sets - most desktop controllers, either RAID or just standard bus chipsets, desktop controllers are never tested or designed to get along with Enterprise drive feature sets. You can see some strange behaviors with them that are totally random. Usually safer, unless the chipset manufacturer recommends a specific Enterprise class drive, to stick with desktop models like Barracuda, Deskstar, Cavier et al.
Drobo = cheap consumer grade junk. Too many of our photographer customers went with them and when they have a corruption issue they then found it impossible to recover. The database is proprietary and the reliability is questionable. In my opinion though, the type of RAID you use is of far less consequence than the type and reliability of the backup. Nothing wrong with a Drobo if your backup is simple and extremely reliable.
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