It's usually very easy to unplug a drive. Just power off the PC, open the case and unplug the cables from the HDD and boot. If you still have issues, reinstall Windows on the SSD while the HDD is still disconnected. As soon as you have a working C drive, you can format the HDD to get some extra storage. If it still feels clunky after a complete reinstall on the SSD (without the HDD connected), then this particular drive could have a problem or a wrong configuration (for example, SSDs need the be set with the AHCI mode in the BIOS SATA options, if it is set as IDE you gonna be stuck with a HDD-like speed).