The base Mac Pro supports three interface options for adding external drives: USB, FW400 and FW800. eSATA is not available, but the Mac Pro has two spare SATA connections languishing on the main logic board. I've subsequently turned these into a pair of eSATA ports and connected them to a pair of Seagate 1 TB drives in a SanDigital MOBILESTOR MS2T+ case.
WARNING: eSATA setup in this manner means these are pretty well considered internal drives by Mac OS X. Apple does NOT support hot swapping of drives connected to the internal ports, so make sure that the MOBILESTORE is powered up when the Mac is booted, and always shut down Mac before disconnecting the external drives.
These drives are setup as two individual 1 TB drives. Initially I set these up as a software mirror RAID 1 with Apple's Disk Utility. I gave up on that after a couple of months and 3 or 4 five-hour RAID rebuilds after one drive would persistently fail out of the mirror. Add that while hardware redundancy is provided with a mirror, so is data corruption. As a backup system, I want a belt and suspenders.
Since the case is simply a JBOD (just a bunch of disks) box, with two eSATA connections which simply connect to both the eSATA ports on the Mac Pro, you don't need to mirror the drives, and they don't have to be the same size. For my needs, adding an external setup that smokes at full eSATA II speed, for about $300 complete, was a nice addition.
Currently one drive serves as my active Time Machine backup for two 500 GB internal drives. Since neither of those drives will ever fill to 100% capacity, I'm not worried about this drive only matching the total size of those it protects and serves. Should that become an issue, I have the perfect excuse to upgrade it to a 1.5 TB drives someday.
The second drive is a weekly clone of the first. I run SuperDuper in the wee hours of Sunday morning and use the Smart Backup option wrapped in a pair of shell scripts that stop and restart Time Machine. Local Time Machine backups tend to have amazing high file counts. With a set going back almost 8 months, the count is about 6 million file references and occupies a mere 700+ GB of space. The weekly update runs out in 3-1/2 hours. Should the primary drive get damaged or fail, I will still have the deep end of my Time Machine storage ready to use on the secondary drive.
If you're interested in the two, one-line scripts that disable / enable TM backups, drop me a note.
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