Upload & Sell: On
Generally raid arrays need to all be the same size. You can put in a larger drive, but it will only use as much as is available on the other drives in the array. So you would need to replace all of them to get a larger array,
I did this a while back with a four drive Netgear array. The process was lengthy but it did work flawlessly.
Which points up one of Drobo's chief selling points: you can expand your available storage space by replacing just one drive at a time with a larger one.
But, however, and this is a big 'but', the 5Ds Jim and I bought are limited to a partition size of 16TB, because who would ever need more than that? I'm not precisely sure what happens when, down the road, we start replacing the 4TB drives with larger units as they become available (right now, with single-disk-fault protection, the 20TB of disk provides 14.89TB of available storage on a single, 16TB partition). Presumably you'd have to repartition the array into a pair of 16TB partitions...but in order to do that you'd have to wipe the data and reload from...where, exactly? I'm planning to use my first (and so far only) Drobo 5D as primary storage, with a farm of individual external disks as its backup, once I finish loading the Drobo from that self-same disk farm. But if I do that, logically, the disk farm should become my off-site backup, which is a PITA.
*sigh*. It never ends.
I guess my next big purchase won't be a new camera or a nice piece of glass, but a second Drobo to use as my offsite backup.