FreeBSD 10.3 on windows 2012 R2 hyper-v may fail to hot add/remove a SCSI disk. The reproduce steps are:
1. boot the system
2. add a vhd disk through hyperv manager settings: From SCSI Controller, add a new virtual hard disk, select vhd and 1G size. Finally click "Apply"
3. From the hyperv manager console, we can see a new disk is added.
4. repeat the above step 2 and 3 twice to add two disks, but you can only see one hot plugin disk. The first hot plugin disk is missing.

If you use "camcontrol rescan <bus:target:lun>" to rescan the missing device, it will go back. But if you used this command issue twice rescan, the disk may miss again.