Deleting disks from pool returns an error

[phillxnet (Philip Guyton)]

@Tomasz_Kusmierz I see from your screen shot that you are running Rockstor version 3.8.15-9.
As of version 3.8.15-22

Dev log for 3.8.16

3.8.15-12 is now available! We are ramping up for 3.8.16 so we are in final regression fixes and testing phase. In this update, three such regressions were fixed – a pool creation bug, pool deletion bug and a samba config bug. We may have a few more to go. Changes in this update are contributed by @phillxnet, @Flyer and myself. Thank you for coming up with these fixes swiftly! Apart from these regression fixes, @phillxnet also fixed incorrect space calculation bug on disk removal.

the relevant calculation has been amended / corrected via the indicated issue and now will only allow disk removal if there exists enough free space on the pool for the worst case scenario of the removed disks being full; irrespective of if this is actually the case. Just to play safe, especially given current space calculation difficulties with btrfs.

Hope this helps.