How to enable a BO in the 'transfer attachments' step in One-Steps

Hey guys,

I feel like I'm missing something really obvious here, but I don't know what it is! I'm executing a simple One-Step to transfer attachments from Incident to Task, however, I can't see any relationships in the transfer attachments step other than the one I'm executing the One-Step from.

e.g.

The only BO I can transfer attachments to, is the same BO I'm copying them from!

I've confirmed that the 'Task' BO has attachments enabled, and that the account I'm using has full rights to import etc. attachments. There is a 1-Many relationship with tasks (I wondered if it needed to be 1-1 or something, but if that were the case then other BOs linked to Incident would have showed up in the drop down instead).

Any ideas / direction appreciated :)

Thanks,

Graham