Cloned SSD suddenly stopped being bootable

I'm trying to wrangle an 8 year old pc into a decently-usable state.
Sequence of events:
1. Cloned Windows 10 Pro 64-bit from HDD to SSD.
2. Bios was being weird about letting me into the boot options, so eventually I try disconnecting the HDD.
3. Success! SSD boots into windows, everything seems fine.
4. Reconnect HDD so I can wipe it to use as storage.
5. System boots from HDD, treating it as C drive.
6. Disconnect HDD again. Reboot. System insists SSD is now not a bootable drive.

I don't understand. At all. It worked and now it doesn't. I even tried re-cloning and immediately bootiing with only the SSD connected, no luck.

Any ideas?

What application did you use to do the cloning?

My default thought when cloning fails is to try imaging instead. Have you rejected that?

I used EaseUS to clone. Like I said it was working, it booted into windows at least once, I just don't get why it would stop being bootable.

Also, you've apparently helped me define the limit of my IT knowledge: I never had the faintest clue that there was a difference between disk cloning and imaging...

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From what I'm reading about disk imaging, I would need a third drive to create an image to? I don't have access to a third drive right now.

I do appreciate you taking the time to reply.

Offhand, I don't quite get why it would stop being bootable either......unless there is more to the story.

Cloning: one step process; the destination drive is immediately bootable if the cloning succeeds. Usually done when all is well and you're just swapping from a working drive to another drive, for whatever reason.

Imaging: two step process; step 1: make an image file and save it somewhere, perhaps an external. Step 2; formally restore that image file to the destination drive, at which point it becomes bootable if successful. There might be 10 seconds between the 2 steps or 10 years. More of a proper "backup" than cloning, but accomplishes the same thing.

I assume you cloned but did not image?

It seems cloning has more frequent issues than imaging, at least as seen on this forum.

You could always try another cloning app....Macrium, Aomei, others that I can't think of right now.

Is your ssd connected via sata or usb?
Old motherboards require a usb drive to be connected with the pc shutdown,then restarted and the bios entered and usb drive searched for and if found,then moved to first boot device,normally through ---Advanced boot options---or something similar.
The original hdd must be disconnected,otherwise it will be first boot..
The original hdd is best connected by usb and the necessary container and cables are available for most popular makes.