Instead of upgrading "internal" factory HDD on the 2010 i3 model, can you install an external HDD via its FW port instead. Simply put "most often used" DATA on the internal drive and "less often used" data on the attached external HDD. For example, external HDD contains documents, movies or picture files that one might load 3 times a year. If automatic DATA backup is needed, perhaps connecting 2nd external HDD to FW (daisy chain??) or perhaps to its USB port will work?
If wondering, I'm uneasy of installing larger size internal HDD inside my 2010 i3 iMac as well. I'd rather connect external HDD - instead of opening up the iMac box.
Hope this helps...