Why did you leave the HDD in the iMac? Take that sucker out and use it as an external drive or sell it on ebay!
Yes, it’s more disassembly, but you’ve already done the hard part by cracking open the screen from the body. It’s not risky unless you’re technically challenged, in which case you shouldn’t open your iMac up anyways.
For those who are comfortable handling CPUs and thermal paste, this is a good opportunity to upgrade the CPU unless it’s already the top option for your generation iMac.