I am in the process of getting one of these right now for my company I work for, they have several flavors (USB, RJ45, eSATA, etc.) of the units, but they use the 3.5" drives, so if you don't mind that too much then these are the way to go! They have an integrated RAID controller which is not seen by the operating system.
I'm looking for something similar but the drives I have are Ultra/ATA (PATA). Question: when shopping for an enclosure, can I use one that supports SATA disks and simply use a conversion cable to mount PATA disks inside... or is this just nonsense?
They specialize in "driverless" Raid1 that is not seen by the operating system.
They make both IDE and SATA solutions in both 2.5" and 3.5" drive sizes. Some are hot swappable. Most of these are directly mounted in the PC but they do make an external unit that uses eSata.
This type of Raid1 can be used with any OS and is generally more reliable than driver based products. Since data is not altered, the mirror drive can also be removed and used as a bootable backup drive.