Does an extra old internal hard disk drive affect my new PC system’s performance?

I've just recently bought a new PC with a high-speed SSD and a hard disk drive.

I do care much about its performance and don't intend to slow it down by any means.

I have an old 2 TB Green Western Digital HDD which has worked for over six years. I just want to use it for archiving. No apps and games and OS will be installed on it.

Will using that hard disk drive affect my new PC's performance?

I understand that you wish to install the old HDD as internal disk inside your new computer.

An unused disk will not affect the global performance, but you should be thinking of other factors: The disk is relatively old (as disks go), so may have a limited lifetime. The fact that it will be powered-on now for some years to come, will use up the spin-time that is still left for it.

This is why I would recommend buying a USB enclosure and installing the disk inside it. The disk will be used as external, and will be turned off when not required (which I understand will be almost always). This way you will extend its remaining lifetime for additional several years.

Overall, no, it will not affect your PC's general performance. Only applications that make use of data stored on that HDD will experience a performance penalty due to the slower data speeds of that harddrive.

Using the drive for cold storage (data that isn't actively being used), which is what you're describing, should just work fine.