Here I would like to briefly report my temporary workaround.
Following the suggestion by @deadite66 , I set the reserved block size of all my ext4 data drives to 0 with
sudo tune2fs -m 0 /dev/sdxx
According to some other users’ remarks I found after quick googling, a low block reserve (as low as 0!) would have no significant negative impact on performance, as long as the concerning drive is mainly for long-term archiving purposes, i.e. without frequent, intensive read/write operations. Please correct me if I am wrong. I have to admit that I don’t have a deep understanding about this topic,
Moreover, mhddfs has a default emergency margin of 4GB, so that the drives will never be actually filled up. The remaining question is, however, what the exact threshold of the low disk warning is? If it is lower than 4GB, than it is all set and I am good to go! But as I implied in my earlier post, my real concern is that I could not find any configuration possibilities of a certain Disk Usage Analyzer, which I believe is responsible for issuing this warning. The graphic is no doubt pretty fancy, but without any apparent setting options, unenlightened users may have a hard time playing with it…
Currently I won’t be able to make any further test either to support or to override my assumptions though, because with the newly released space, I have to wait until they get filled up again;) But at least for the time being, the annoying low disk space warning is gone ~