Why is my HDD slowing down my PC?

OK, so I've got a Toshiba 1TB HDD, but my old HDD (WD Caviar Blue 250GB SATAIII) slows down my pc everytime I plug it in. I plug in the SATA, the SATA Power, but when I try to open it anywhere, Windows Explorer crashes, Diskpart is not listing the disks, and Windows installers (I tried wint 7, 8.1 and 10) don't load in, and if I try to boot from it, it says that "Non-System Disk or harmed disk. And when i try DPS Self-Test in BIOS, it says that " disk replacement recommented", and I don't know what to do with it, because some important files are on it.
Thanks for any help!

Welcome to the forum.Does the bios see it correctly if disk manager isnt seeing it its not good. Is it a bootable disk?

If your computer is telling you the drive is bad, then very likely the drive is bad. What to do with it? Get a replacement for it and copy your files as soon as possible before the section of the drive that contains those files goes bad.

It is dead, worn out.
You should always backup important files on any storage medium, to another drive.

Rescue what you can as soon as you can on to a new drive, and hope.

Samuria said:

Welcome to the forum.Does the bios see it correctly if disk manager isnt seeing it its not good. Is it a bootable disk?

Yes it is, the system was on it for 1,5 years

First backup your critical data as noted above.

Then run chkdsk /r to fully check & repair the drive. Also see this link and check the drive with the WD diagnostics https://support.wdc.com/knowledgebas...px?ID=14920&s=

Having two drives with active boot partitions can sometimes confuse the Windows boot process. Try marking any active partitions on your external HDD to inactive using MiniTool Partition Wizard