I have installed a 10 Gb Toshiba MK1011GAV drive which is not on the Roland approved media list. Nevertheless it works fine, and installation took less than 10 minutes (Initialising took quite a bit longer, though!).
By the way, the maximum capacity a VS-1680 can use on any disk is 8 partitions of 2 Gbyte each, so that's 16 Gbyte per disk. However, you should be aware that diskdrive manufacturers are a bit more optimistic about their drive capacities than Roland, Microsoft and most other people, so on a drive advertised as 16 Gbyte you can fit only about 7½ partition (the last being only about 1 Gbyte). If you buy larger (i.e. 20 Gbyte), you'll just waste some capacity, but it should work fine.
Should you require even more, you need to use more disks, which will need to be external SCSI ones. The narrow SCSI-3 bus used by the VS-1680 can address 8 devices, but it's own SCSI controller will use one of those. So if you just want to use disks you can use seven, but only six if you still want to use a CD-R drive. This will still give you plenty track minutes...