Hard Disc Drive tainted by new system, prevents booting in old system

[XGwolf36]

I have an Old gaming PC, and a new gaming PC (Specs below).
In my old pc, I have windows on a small SSD, and my games on a HDD.
In my new system, I have just one large SSD.

I physically moved the HDD from the old PC to the new PC, and transferred some steam game files to large SSD drive.

The HDD has been installed in the new PC for a couple of months. I have played games from the large SSD and also had updates.

As far as I know, my new PC hasn't actually done anything on the HDD except for "Copy".
Today, I moved the HDD back to the old PC, because I want to uninstall everything.
When I boot up, the screen is black and it says "A2" in the bottom right corner of the screen, which relates to an issue with a storage drive.
I unplugged the HDD, booted the old PC up and it worked fine, straight into windows.
Plugged the HDD back in and again the A2 error.

The HDD is preventing 'posting' in the old pc.

 

What in the actual fruit cake is going on? Pls help
My intention is to delete things so If I end up wiping the drive that's not so bad.
 

Old PC:
 

OS: Windows 10 Pro
CPU: Intel core i3 4170
Mobo: MSI H97M-G43 M-ATX Motherboard
RAM: 8GB DDR3 1600MHz Kingston Hyper X Genesis
GPU: Asus Radeon R9 270x 2GB
Boot Drive: Corsair 60GB SSD SATA 2.5 inch
Extra Drive: Seagate Barracuda 1TB HDD Sata 3.5 inch
Power: Silverstone Strider Essential 500W 80+ Bronze (SST-ST50F-ESB)
 

New PC:
 

OS: Windows 11
CPU: AMD Ryzen 5 5600 3.5 GHz 6-Core Processor
Mobo: MSI B550 PRO-VDH  Micro ATX AM4 Motherboard
RAM: Corsair Vengeance LPX 16 GB (2 x 8 GB) DDR4-3200 Memory
GPU: Gigabyte EAGLE Radeon RX 6600 8 GB Video Card
Boot Drive: Samsung 970 Evo Plus 2 TB M.2-2280 PCIe 3.0 X4 NVME Solid State Drive
Extra Drive: None
Power: Silverstone Decathlon DA650 Gold 650 W 80+ Gold Certified Fully Modular ATX Power Supply

[DesignDecay]

Hold F11 when you turn on your computer. It should open into a menu that will allow you to select your boot device. Choose your SSD and it should boot.

 

Assuming that works reboot into your bios and look for a setting that says boot order. 

Change your SSD to the first position.