Strange one today.
MDT integrated Task Sequence, version 8443, running in ConfigMgr (1610) kept creating a BDEDrive with an assigned drive letter when building a device using legacy BIOS.
I’d ticked the usual checkbox in the BDEDrive Partition properties, ‘Do not assign a drive letter to this partition’ but a drive letter was assigned with every build.
The fix was to untick the box!
Not seen this behaviour before. I would love to know if others have experienced this. Let me know if you have.
SCCMentor - Paul Winstanley 
We are also getting the same issue.However, it is random but will try un-ticking the checkbox.
We were getting this issue using SCCM 2012 R2 SP1. We have recently migrated to 1606 and soon will be upgrading to 1702. Will try this option with both versions.
Interesting. Is it using MDT 8443?
We are using MDT 2013
I had the same issue, what a weird fix. This is just typical microsoft.
Good to know that wasn’t just me going stir crazy. Cheers Paul
We had the same problem. A new deployment of Windows 10 left the system with the BDEDRIVE assigned to D:.
After unchecking te Box in the TS it all worked well. No driver letter assignment.
Thanks for the tip.
System:
SCCM 2012 R2 SP1 version 1702
MDT 2013 Update 1