Macintosh bootable volumes are supported on the internal 2-port PCI card (part number TSATA) but are not supported on any of the other SATA cards at this time.
This does not happen often, but sometimes the driver caches do not get purged and the new SATA driver does not get loaded correctly from ROM.
Try this:
1) Delete and empty trash for the following file and two directories:
/System/Library/Extensions.mkext (a file)
/System/Library/Caches/com.apple.kernelcaches (a directory)
/System/Library/Caches/com.apple.romextensions (a directory)
Sonnet has tested the following cards with the Drobo S:
Tempo SATA E2P..............................compatible
Tempo SATA E4P..............................not compatible
Tempo SATA ExpressCard/34...........compatible
Tempo SATA Edge ExpressCard/34...compatible
Tempo SATA Pro ExpressCard/34.....not compatible
Tempo SATA X4P..............................not compatible
Qio eSATA ports...............................not compatible