Its really major issue and I simply can’t imagine how this went through the testing phone unnoticed. Here’s the problem:

If you have the Messaging Memory set to the N91 HDD, then you can’t use the phone as a Mass storage device.

I recently switched messaging memory over to the N91 HDD as I was running low on phone memory. And since then, I haven’t been able to use Mass Storage at all. It says, “Messaging is currently using the hard disk. Mass Storage mode cannot be activated”. I use a powerbook, so mass storage is the only option for me when I transfer music. So for now, I have to choose between being able to access and save my emails or being able to transfer new music to my phone.
This totally sucks! I really hope someone from nokia reads this and releases a patch.