Ok, the ex-wife bought a drobo and fitted it with three 1TB disks, copied the data off her existing 1.5TB drive onto the drobo and then added that 1.5TB disk in the fourth slot.
The expected behaviour was for drobo to expand onto the fourth disk giving an increase in available space. What actually happened was that the drobo dropped the first 1TB disk and rebuilt itself (taking about 20 hours) onto the three disks of 1+1+1.5
It’s finished the rebuild now. The dashboard does not see the first 1TB drive and there’s no light next to it on the physical unit. The drobo is at current fimware on a Mac.
I expect that the correct thing to do now will be to remove the 1TB disk thats not in use, wait a few moments and then put it back in, which should trigger the expansion, but could someone confirm or enlighten on this point please?