VDP failing claiming not enough space....I don't believe it

2016-02-02T20:00:16.719-01:00 avvcbimage Info <14625>: WorkOrder Disk Info:

  Prior Disk '2000': file(base):'[wb_prod_ql01os] WB Prod QL01/WB Prod QL01.vmdk', backItUp=1

               snapshot file:'[wb_prod_ql01os] WB Prod QL01/WB Prod QL01.vmdk'

               prior size(KB):0, current size(KB):57671680, match=0

               prior change block ID:''

               Datastore:'wb_prod_ql01os' Directly Accessible=1

  Prior Disk '2016': file(base):'[wb_prod_shard01] WB Prod QL01/WB Prod QL01.vmdk', backItUp=0

               snapshot file:'[wb_prod_shard01] WB Prod QL01/WB Prod QL01.vmdk'

               prior size(KB):0, current size(KB):1572864000, match=0

               prior change block ID:''

               Datastore:'wb_prod_shard01' Directly Accessible=1

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SNIP

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2016-02-02T20:00:20.265-01:00 avvcbimage Info <19704>: DataStore Storage Info:wb_prod_ql01os capacity=90999619584    free=30713839616

2016-02-02T20:00:20.265-01:00 avvcbimage Info <19716>: DS Capacity=90999619584    FreeSpace=30713839616    / HD committed=59255284393    unCommitted=594            un

Shared=59055800320

2016-02-02T20:00:20.289-01:00 avvcbimage Info <19704>: DataStore Storage Info:wb_prod_shard01 capacity=1675037245440  free=63387467776

2016-02-02T20:00:20.289-01:00 avvcbimage Info <19716>: DS Capacity=1675037245440  FreeSpace=63387467776    / HD committed=1610612736000  unCommitted=597            un

Shared=1610612736000

2016-02-02T20:00:20.290-01:00 avvcbimage Info <19717>: DS(wb_prod_shard01) does not have enough free space (63387467776   ) for disks used (80530636800).

2016-02-02T20:00:20.290-01:00 avvcbimage Error <19661>: Datastore does not have enough free space for snapshot

I've pasted prudent entries from my VDP appliance log file above.

Basically I am backing up a single disk on a 7-disk VM. You can see in the first log entries above (before the SNIP) that disk 'wb_prod_ql01os' is marked for Backup (backItUp=1), then the next disk, 'wb_prod_shard01' is not marked for backup (backItUp=0).

We then go down the logs (below the SNIP above) and see that VDP has failed due to 'Datastore does not have enough free space for snapshot', but it's pointing at 'wb_prod_shard01' as the culprit! That disk is not marked for backup so why on earth is it causing VDP to fail??

Any help hugely appreciated