We all know we can add vdisk to a guest LDOM while online.
On the other way round, can we remove a vdisk from a guest while online?
I got the following when trying to remove the vdisk, I ensure the disk is free and no file system mounted.
Failed to remove vdisk instance
The -f option has no effect because the device cannot be removed safely
Yes, Virtual IO DR works for both adding and removing virtual disks. You also seem to have done the steps necessary to make sure the vdisk can be removed (making sure it is unmounted and not in use).
I'd suggest checking /var/adm/messsages on the guest and see if any of the kernel drivers reported an issue or something holding the disk open for some reason. Also, see if there are an message in the ldmd SMF log (/var/svc/log/ldoms-ldmd:default.log)
(and checking that 'drd' is online in the guest is always a good thing to double-check)