I have exported a list of accounts with account owners and other info from Production and have reassigned accounts to new owners in excel. I would like to now use the Data Loader to push these changes into the sandbox and eventually move the changes to production. After downloading Data Loader for Mac I got an error when I try to open it saying "Data Loader is Damaged and Can't Be Opened. You should move it to the Trash" How do I fix this? Or is there another tool I can use to reassign accounts based on the excel file?
Use https://dataloader.io/ . All you need to do is log in with your org credentials...It's pretty fast and more efficient than the Apex Data Loader. As you are saying it's very important to get this job done, this is would be the best way.
Actually, why don't you use Mass Transfer Records feature? That's the efficient way of transferring records to a different owner. You can transfer 250 records at a time. Unless you have more than 2000 records to transfer, Mass Transfer Records is the best way to do. In the Set Up search box, search for Mass Transfer Records....it is pretty straight forward and nothing confusing even for a first timer....