Please see my responses in Red
and thank you for the reply.
I'm not really sure I can help you in resolving the problem you've run into. We've been having some problems dual-booting Win 10 and Win 7 and/or Win 8.1 also and I'm unsure if the problems we've experienced are relevant to your situation.
I wonder if you would be good enough to provide information concerning your multi-OS configuration, along the following lines...
1. Obviously the dual-boot setup was created on the 256 GB SSD and presumably there were no problems booting to either Win 7 or Win 10, right? Correct
2. Can I assume the 256 GB SSD was MBR-partitioned? Correct
3. Presumably there were two partitions set up on the SSD, each one containing an OS. (I realize there might have been other partitions, e.g., System Reserved, UEFI perhaps, System Recovery possibly, but those are not relevant). Correct
4. Was the Win 7 in the first partition and Win 10 in the second partition? Correct
5. How were the two OSs installed in the first place? Was the Win 7 OS fresh-installed or was it cloned from another drive? Windows 7 was preloaded from Dell
6. Was Win 10 fresh-installed in the second partition or was it also a clone? It was fresh installed on the new partition
7. How did you manage the multi-boot aspect? When you booted the PC, did Windows provide a menu (option) that allowed you to select either Win 7 or Win 10? Boot Menu with Windows 10 as default and Windows 7 as the second choice
8. Or did you utilize Disk Management to "Mark Partition as Active" (selecting the OS on the partition you wanted to boot to when next you powered-up the PC)? (I'm again assuming the disk was MBR-partitioned). N/A
9. Or did you use a third-party boot manager to determine which OS to boot to? If so, which program?N/A
If it's not too much trouble I'd appreciate receiving your responses to the above.