Help ,think I broke my computer

Please don't try the intended HDD storage drive internally again until after reading.

This has been a challenge for others also.  If you can't get your way in BIOS or the Boot Menu...

First, a tip:  In BIOS boot options, either Windows Boot Manager should be checked off or the m.2 SSD.  With the Optiplex being a business machine, and it was bought refurb or used, there may be other boot options that need unchecking or deleting in BIOS that are related to booting from a network.  Might start with something like IPV4, IPV6, etc.  If there's things in the boot option list not described here, could you please upload a photo of it?  Next...

I'd mount the HDD externally in an enclosure or dock with on & off switch.  With it off, boot your PC from the m.2 drive.  Then turn on the HDD, go into Disk Management, and use reformatting to wipe the drive.  Even if it's formatted to NTFS, reformatting it again to NTFS (and use GPT partition scheme) will wipe the drive.  It's now ready for storage.

Before wiping the HDD, if PC doesn't see the HDD when powering it on, simply use Update in Device Manager.

This is no guarantee that the PC might not still try to boot from the HDD once reinstalled in PC, but I don't see why it would   If that's the case, you have an external storage drive.

"Not sure its worth spending extra money on but if I were to connect it externally via USB but used a different PC would I still need a switch."

Please pardon, I'm not 100% sure I understand what you mean by your last sentence.  I think what you might mean is wipe the HDD on a different PC?

External enclosures and docks typically come with a switch.  Here's what I use:  Sabrent external HDD enclosure on Amazon  And the price certainly went up a little since I bought mine last year, but still not too bad.  Ugreen costs more.

If you plan on wiping the drive on a different PC that boots normally, but your HDD is still a boot drive not wiped yet, yes, you need to use the switch like I described above.  Once it's a storage drive, you'll be able to power it on first, then boot PC.  Or try to go back to using it internally like Chino said.

There's also this option:  If you decide to wipe the HDD in another PC, install it in the PC.  Upon starting the PC, immediately press F12 repeatedly for the Boot Menu.  Then choose which drive to boot from.  Then use Disk Management to wipe the HDD you want to wipe.

After it's wiped, install it in the PC with the m.2 drive.  PC should boot normally without having to use the Boot Menu each time.  If not, then external enclosure for storage drive is the way to go.

The adapter linked above won't work.  USB has 5v and HDD's are 12v.  That's why external power is needed if using HDD externally.  It does work with SSD's because they're 5v.

The advertisement does mention the adapter being compatible with 2.5" HDD's.  They got me on that one because I thought 2.5" HDD's are still 12v.  May be laptop HDD is 5v?

Right; then the adapter you linked will work.  If before wiping HDD, just remember not to connect HDD to USB until micro (or other PC) is fully booted.

Please let us know if it all works out?

Something I remembered is if 2 boot drives with the same OS are in 1 pc, namely Win10/11 (not sure about Win7) Microsoft doesn't like that.  The problem I had is 1 drive would sabotage the other, and I'd get unrecoverable BSOD.  Would have to wipe drive and start over.

I run an older version of Win10* on 1 of my boot drives since 22H2 (and a few prior versions) has a few bugs I don't like.  In turn, boot drives are in hot swap SSD/HDD bays with switches.  Storage drives are external with switches.  Can only use storage with 1 of the boot drives, otherwise, Win10 will use them to detect 2 Win10 drives in the same PC.  *To keep an older version of Win10, PC is only online when I want it to be.