How to format a drive encrypted with Bitlocker?

Press SHIFT-F10 or hit 'repair' in from the Windows installation to open up the command line, then execute the diskpart command and delete the partition, e.g.: list disk, select disk 0 or any other identifying the correct disk, list partition, select partition 1, or the encrypted one , in case there are multiple partitions, then delete partition override.

You can then resume the install procedure normally to repartition and format the drive.

I figured it out - You do this:

1. Hit Win+R to open the run dialogue box and type diskpart and hit “OK” to open a black command prompt window.
2. Type list disk to display all the disks of your computer.
3. Type select disk n. Here n stands for the disk you want to work well.
4. Type list partition to display all the volumes on the hard drive.
5. Type select partition n. Here n stands for the volume you want to delete.
6. Type delete partition override to get rid of the volume.
7. Type exit to close the window. This will wipe all the data on the USB/Partition.
8. Take your USB out and put it back in again.
9. You then need to go into Computer Management > Disk Management
10. Right click the USB/partition > Make new volume.
11. Then follow the prompts.. Next > next > next - then your done!

Start a Linux Live CD like the one from GParted and delete all (Bitlocker enabled) partitions.

If that doesn't help there is still the possibility to wipe the complete disk - after overwriting the first few megabytes the HDD will be recognized as fresh new HDD by Windows. You can do that for example using DBAN but don't forget to disconnect all the other HDDs before using it - otherwise you may delete the wrong HDD and loose all your data.