I think that my Fenvi was the 9.5mm, as it is really as slim as the HD to be inserted in it, plus the thickness of the back aluminium foil.
I do not understand these problems about the HDD "spinning up", or having problems with standby. On my machine, the hdd is automatically switching off after approximately 5 minutes of non usage. It wakens automatically when you need it, in a couple of seconds. No need to unmount and remount...
And the machine goes to sleep nicely simply closing the lid, and wakens up instantaneously when opening the lid.
All of this happens both on OSX-Lion and Windows7-64bits.
Please note, however, that my HD is a purely-data disk, as both my OSs reside on the 240GB vertex SSD in the main bay.
Regarding the exFAT format for the HD. I got another couple of times the messing up of the volume due to misbehaving apps under OSX, but till now everything was fixed easily by rebooting in Windows and issuing the CHKDSK /F command... Most of these messing up occurred using Parallels, so at the end I removed it. Now, when I need to rum some specific Windows-only app which I am missing under OSX, I launch them through Wine, thanks to the excellent WineBottler free app. This is working fine for Adobe Audition 1.5, TotalCommander, PaintShopPro, etc... I suppose that, as soon as these apps (Winebottler, Crossover) become mature enough, there will not be any need to maintain a dual-boot machine and to pay for a full license of Windows...
This will mean that it will be not necessary anymore to waste so much time for setting up a complex dual-boot system as the one which I have now.
On the other hand, I cannot really recommend to anyone to switch "completely" to OSX: without the capability to run some specific Windows-only apps, the productivity is definitely reduced - and I do not see any chance that these beloved, critical apps are ever ported natively to OSX...
In my opinion, Apple should address this fact natively, inserting Wine in OSX and developing a "bottler" app working better than WineBottler (or Crossover, which appears to be better, but not perfect yet). Only this way OSX can catch the multitude of Windows users, who cannot renounce to their favorite Windows-only apps.