It sounds to me like if there is adequate memory, then the additional pack will install, if the memory is near full or needs defragging, then it will format the memory and you will loose the previous installed packs......
I've tried 3 options, install only one pack at start, several packs together, then add a single pack. YEM format the memory every time, even if it's not used 25% of the capasity.
So everything you want to install of style/soundpacks into expansion memory has to be marked and install in one go by use of YEM.
It's a bit strange, imagine have to do similiar when add sofware to your PC, HD get formatted and all content has to be reinstalled in one go.
If this should work as it did at T4 extended memory, you could add one pack at the time without any loss of already installed stuff, I guess Yamaha will fix it in a future update.
Anyway, if this for some reason or another is intended to be like this, it would be interesting to know why. Could it be the type of memory used that has to be 'flashed' in one go?