Can I separate and move the boot partition from an old HDD to a smaller HDD or SSD?

Switching Windows Boot Manager from HDD to SSD

(1) It is the EFI partition that holds the boot files including the BCD (Boot Configuration Data). The BCD tells the boot process which Windows partition to boot to. It looks like you set the BIOS correctly to boot using EFI (& its BCD) that is on the SSD. However, that is a clone of EFI on the HDD — which loads the Windows partition that is on the HDD. You need to alter the SSD’s BCD to boot to the Windows partition that is on the SSD. Let’s see your BCD…

(a) Type CMD into Search, & click Run as Administrator.

(b) Enter the following commands…

BCDEdit /Enum All <<<Show the BCD

DiskPart <<<Enter DiskPart

List Vol <<<List the Volumes

Select Disk 0 <<<Focus on the HDD

List Part <<<List its partitions

Select Disk 1 <<<Focus on the SSD

List Part <<<List its partitions

Exit <<<Exit DiskPart

Use Ctrl-A & Ctrl-C to copy, then post it to us. We can adjust it manually.

(2) It might be as Paloma suggests, that disconnecting the HDD will cause the BCD (on the SSD) to self-adjust. Windows might do an automatic Startup Repair. But I hope it also gets the Recovery partition right.

(3) I guess you want that D: partition. So, after all is well with the SSD, you may plug the HDD back in. But it will be best to delete its EFI, Windows & Recovery partitions. Just keep D:.