Aurora R12, adding an HDD from an old Inspiron

No, that's not going to work -- the system will not boot from that drive, which is required to run programs from it.

If you want to run those programs, look for a working, used system of (preferably) the same model - or one as close as you can find.

Otherwise, the programs will need to be installed on the new system (probably necessitating updates or upgrades).

If your old Dell HDD has Windows 10 installed via UEFI bios, you can try install it as a dual UEFI boot drive in R12 and press F12 on power on to select which UEFI drive to boot from.  R12 booting from old Dell hdd may take a bit time to install R12 drivers on it and hopefully would not crash.  

If old HDD has legacy bios installed OS, then you can not boot from it in R12.

what ejn said is that you can not run programs from old HDD while booting from new boot drive.  the programs that can run n HDD have to be originally installed in the same OS environment.  so if you boot from new HDD in R12, all the old program on the old HDD appear just like random data without any meaning to OS.

If the system was 10 years old, it's all but assured it is a legacy boot.

You would need to remove the factory installed SSD in order to boot any SATA hard drive in the system.

a 10 year old Dell is from 2015 approx.  If it were from 2009 then yes definitely OS is legacy only.  On the other hand it were a Inspiron 3847 (circa 2013) or later it would have options of either legacy or UEFI bios.  you could just give it a try does not hurt in R12.  If it is UEFI, it would try to boot from it if you press F12 and select the HDD as UEFI boot drive.  If it crashes, try again.  does not hurt.  If it would not boot in the end, remove it, you could still boot from new HDD when it is the only drive present in R12.

It's not said what OS is on the old and new drives.  If both drives have Win10 for example, Windows absolutely figures it out and will BSOD one of the drives.  It starts with "disk checking."  This only happens if both drives have power while booting from one or the other.  I went through quite the ordeal to figure it out.

I have Win10 on 2 drives in order to keep version 1809 on one of the drives.  Both drives are in hot swap bays with switches.  That way, only one at a time has power.  New Windows versions have bugs I don't like.

All data and files should be backed up outside of PC.  However, if there's any data or files you want to get off the old HDD, Windows won't BSOD right away.  While booted from the new drive, it does give the chance to get files off of the old HDD.  Then disconnect old HDD when done.

You're welcome!!  :)

""If old HDD has legacy bios installed OS, then you can not boot from it in R12" How would I know if this was the case?"

Check BIOS settings.  If it's set to UEFI, and old HDD boots, then it was loaded in UEFI mode.  The same for Legacy.  If it was loaded in Legacy, and BIOS is set to UEFI, it won't boot, and vice-versa.  I'm not sure where "R12" came from.