Are the correct USB drivers installed on your client's pc? And by that I mean effectively the manufacturer's driver (especially if it's an external usb 3 chipset) and not just the driver that Windows installed automatically when it was first installed?
I'm asking because I once have solved similar issues by doing this. Took me ages before I found the problem. Granted, this was not with a Drobo, but the problem was very similar.
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup