Q - Is it possible to restore the original MBR that uses the key from the backup file (HPDriveEncyrptionBackup.dat), with which the remaining two partitions are still encrypted, so that the two remaining partitions are instantly accessible again? My guess is that this is definitely possible using a disk edition given the exact location of the encryption key on the disk.
A - No, sorry, not with the HP product. It's not a single sector either - there is megabytes of data you'd have to reconstruct. It's simply not possible to connect a drive from one activation to either another activation, or a new activation.
Q - Alternatively, is it possible to speed up the decrpytion process? I have no clue what the bottleneck might be, it is definitely not CPU or HDD since fans remain silent during decryption and HDD speed is only about 2500 sectors (which should be ~1MB) per second.
A - If you are using BartPE the encryption/decryption should run at the same speed - if they are not, it usually means you don't have the right hard disk controller drivers running in your BartPE image.
Thanks for your answer!
I noticed that the regular copy speed in BartPE is quite high (used the copy command from the shell on a larger file). I guess it is at the same level that can be achieved in windows 7 (>80 MB/s). Could it be that the safeboot driver is somehow accessing the hardware directly by bypassing the SATA driver?
One final question to prevent future hassles with the drive encrpytion:
How to identify the sectors that need to be dd'ed (raw copied) to restore the decryption mechanism once it got corrupted?
no, it's not possible for the EEPC driver not to be using the ATAPI stack in Bart PE.
And it's also not possible to restore the encryption environment. If that happens, use the standard EEPC tools to recreate it. You can't solve problems by copying sectors.