First, I think I need to tell you some basics.
In the Legacy setup like your, a disk that have the OS (windows 10, specially) installed has two partitions. One is System Reserved, of 450 MB and the other is C (or may be any other user defined drive letter).
The System Reserved 450 MB (unlabelled; so not visible in windows explorer) partition is must to be Primary, Active and having the Boot Flag. It contains the bootloader entries only.
The other partition (generally C) contains the OS. It should not be active, does not need the boot flag.
The 450 MB partition boots the computer to the OS (C). That means, The Boot partition (which must be Active) runs the System partition.
Here is a typical example of the disk structure as Partition Wizard sees ....
Now, if the boot entries are corrupted, Rebuilding MBR or running startup repair is enough to repair them and can make the computer bootable.
If boot entries are totally missing, rebuilding MBR should be tried. If it is not enough to do the job, run startup repair for three separate times. This will record the boot entries in the disk properly.
*To get back where you were, connect all the disks. You know that the external (unlebelled) disk has the boot loader. So set all the partitions inactive allover all the disks other than this one. Keep this one as active. Now, you have only one disk with an active boot partition.
Now try to boot the computer normally.
If it fails, rebuild MBR of the unlabelled external HDD.
If it fails, run startup repair thrice. It will make the computer bootable again.
To have the permanent solution later on, do it ....
If you want to start afresh, then ......
- First choose a disk on which you want to install windows.
- Format both the other 256 GB disks, the internal one and the external one. Make it sure that none of them contain any Active partition. Set all their active partitions as inactive.
- Disconnect all the HDDs other than the chosen one. Perform a clean install of windows on it. As you have already upgraded to windows 10, the motherboard already got the digital entitlement; so activation would not be an issue.
- Add back the other drives.
If I have been in your place, I would have taken this way at this stage.