Select second boot drive

[VH2000]

Your Partition E is the boot partition for WIN 10 but it should not have a drive letter! and your WIN7 installation does not have a boot partition. It is implemented in C

Try this:

disconnect the WIN7 disk and check if you can boot from your E-Partition

[VH2000]

OK then type:

For backup your BCD

Create a folder in F "BCD_Backup"

bcdedit /export "F:\BCD_Backup\BCD Backup1"

then type:

bcdboot F:\Windows

This creates an additional bootloader entry.

Edit:

You have 2x 2,5" disks. That's good

http://h10032.www1.hp.com/ctg/Manual/c03099448.pdf

[Dietmar2021]

Wow! Thank you very much VH2000. Dual-boot works now as we wanted it to do.

The partition System-reserved (E:) is still shown. Can I delete partition E:?

I would like to send you a small appreciation. In which coutry are you? Do you have PayPal?

[VH2000]

Nice to hear! 

But I am not so happy with the solution.

I would make the WIN 10 disk as my primary boot disk. There you have a better possibility to make boot settings because it has a separate Boot-Partition. (E:) 

This is better for troubleshooting boot issues! But never (!) delete that partition.

[Dietmar2021]

How would you make the WIN 10 disk as the primary boot disk? By changing HDD 2 from SATA-Port 2 to SATA-Port 1 and HDD 1 from SATA-Port 1 to SATA-Port 2? Or by changing any boot parameters?

[VH2000]

OK, if there is no option in the BIOS F10 to select the drives you change the SATA-Ports first. Then boot from the WIN 10 disk and keep the WIN7 disk disconnected.

This registers the WIN 10 installation to your NVRam.

When ready and it boots without problem you can connect the WIN7 disk.

When ready come back for the last step and a screenshot with the diskmanager

[Dietmar2021]

- change the SATA-Ports first -> done

- boot from the WIN 10 disk and keep the WIN7 disk disconnected -> done! system booted

- When ready and it boots without problem you can connect the WIN7 disk -> done! No more dual-boot selection

[VH2000]

Strange there is still the drive letter (E:)

run cmd (admin) and type 

diskpart

sel disk 0

list par

sel par 1

remove letter="E"

exit

exit

did you connect the win7 disk?

It should be in the diskmanagement

[Dietmar2021]

I have executed the following commands as admin:

diskpart

sel disk 0

list par

sel par 1

remove letter="E"

exit

exit

Result: -> drive letter (E:) is no more there

did you connect the win7 disk? -> Yes, it was connected and it is still connected

[VH2000]

That looks good !

cmd (admin) copy &paste

bcdboot F:\Windows

bcdedit /set {default} description "Windows 7"
bcdedit /set {current} description "Windows 10"

bcdedit /displayorder {default} /addlast

Reboot

Enjoy