Please don't confuse 'spindown' with 'powerdown' - they are two different things - there is still power to the drives when they are 'spundown'.
Having said that, the SATA connector was designed for 'hot plugging' and your only risk in pulling a drive with the unit on is loss of data - so yes, you can pull the drive, with minimal risk of data loss, whilst the drives are 'spundown', but since the unit does not support 'hot swap' which is different to 'hot plug', the question becomes - why would you want to, what is there to be gained by taking the risk?