I currently use a roughly 480GB SSD for Windows 11 File History, but it is running out of space and I cannot afford a new 1–2TB SSD right now. I have a couple of 1TB SATA hard drives from around 2009 that have been stored unused since about 2012. Would using one of them for background backups noticeably affect everyday PC performance, or would I mainly notice the drive spinning up and making noise?
3 Answers
An external enclosure or drive dock is a practical option. You can connect the disk only when you need to run a backup, which avoids constant noise and protects the backup from accidental changes, malware, or a failure in the main PC. A network storage device with cloud synchronization would be a more convenient long-term setup, but the old HDDs are perfectly reasonable as a budget solution.
A hard drive should work fine for File History. Backups will take longer than they would on an SSD, but once the initial backup is finished, the ongoing changes are usually small. You might notice some disk activity or noise while it runs, but normal desktop use should remain responsive. Scheduling backups for times when you are away can reduce the annoyance.
The bigger concern is the age of those drives, not their speed. Test their health with a SMART utility and make sure they do not report bad sectors or other warnings before trusting them. Also remember that a backup kept inside the same computer can be lost along with the computer, so an external or separate copy is useful for anything important.

If the drives are only used for backup, slower transfer speeds are usually an acceptable trade-off. I would still keep another copy elsewhere for irreplaceable files.