Thanks for your post.
I've got a DS411+II and I installed a 500 TB drive and than added a 1 TB drive, the expansion was finished very fast, don't know now, but not longer than 1 hour.
Than I tried to add another 1.5 TB drive, startet the expansion, but nothing happend, it only displayed "Expanding (unmounting volume)". I shut it down restarted it and everything worked fine (the volume was not expanded!).
Now I've installed the system again DSM and HDDs everything new.
Yesterday I got another 1 TB drive and started the expansion, but until now (1day + 5hours) the system is always "Expanding (unmounting volume)" ...
No percentage - nothing, the fans are on - the disks are silent, the LEDs just blink when I access the DSM Manager ...
Don't know if it is working or not
The CPU level and the RAM is nearly unused.
See screenshot, might be a bug (should be faster - but maybe this takes that long?).
I let it "work" until tomorrow and than I will see - or not