You have to remember the PERC 5/i and all other server-grade RAID cards isn't just your average controller. It's got a NVRAM that stores drive configuration. What that means is every time you plug in a new drive that's not part of an existing array, you need to setup it up. And when you remove it the controller will remember it as a failed/missing drive.
Basically in short, you can't treat it as a hot plugging eSATA.
If the motherboard itself has SATA ports, you can use those instead as eSATA.
For backup, most do it over Ethernet to a NAS.