You run the BitLocker Disk Prep tool and you get the error "The bitlocker Drive Preparation Tool could not find a target system drive. You may need to manually prepare your drive for BitLocker."
The BitLocker Hard Drive Preperation Tool BDEHDCFG.EXE is used to repartition disks to make them BitLocker friendly. I have tried this tool to partition a group of Windows 7 laptops for MBAM and for a little while it didn't seem to work, it gave me the error "The bitlocker Drive Preparation Tool could not find a target system drive. You may need to manually prepare your drive for BitLocker."
The support team wrote this article to resolve this problem and it should work: https://support.microsoft.com/kb/933246
However, if it doesn't work for you, here's what the problem is. The tool is trying to write the boot files to the new system drive, but you might have applied an MBAM policy to prevent write-access to unecnrypted datadrives. With the data drives being unencrypted the system volume will be created but the tool will tell you that the drive is write protected This is due to the write-access prevention policy you created earlier for MBAM.
1- disable the GPO link that configured MBAM
2- restart the machine that you want to create the drive on
3- run the prep tool again
now it should work,
once it has worked, you can relink the MBAM GPO and run gpupdate /force