Upgrading software to macOS Big Sur

I don't know which modules spec is right for your Mac. Just go to either Crucial.com or MacSales.com and find your model. Each site will have options for different amounts of RAM in pairs. If you're unsure contact the customer support for the sites and have them find the correct modules.

➡️ +1 - I agree you need more RAM from Crucial or OWC.

➡️ However, you Mac is also slow due to the 7200 RPM hard drive. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Jack

Please never share personal information on a public website. In your screenshot you included the serial number of your computer, I have asked the moderator to remove it. Other types of PI include:

• Addresses (street or virtual)
• Phone numbers
• E-mail address
• Serial Numbers

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

No, you cannot upgrade the 2013 27" iMac to macOS 11 Big Sur.

The best that iMac can do is Catalina 10.15.7

These Mac models are compatible with macOS 11 Big Sur :

• MacBook (2015 or newer)

• MacBook Air (2013 or newer)

• MacBook Pro (Late 2013 or newer)

• Mac mini (2014 or newer)

• iMac (2014 or newer)

• iMac Pro (2017 or newer)

• Mac Pro (2013 or newer)

I don't see any of the usual suspects.

What is slow about your Mac? Is it boot time, application launch time, file open and save times or processing times when in apps with multiple files or windows open?

You have the minimum of RAM needed to run Catalina. If your slowness is with open files, i.e. processing images, or browsing with many windows open then additional RAM can help you. You can add a pair of 4 GB modules for a total of 16 GB or a couple of 8 GB modules for a total of 24 GB.

If your problem is more boot time and application launch time then booting from an external SSD that you've cloned your internal boot drive to would probably help. If you consider an external SSD I suggest you contact OWC (MacSales.com) customer support to find out which of their external SSDs would best suit your iMac model and needs.

Also did you install any of the software that came with the WD and Seagate drives? If so it's not needed and can interfere with the system's management of the drives. They should uninstalled according to the developer's instructions.

Lunuarte wrote:

if i add a pair of 4 GB modules for a total of 16 GB or a couple of 8 GB modules for a total of 24 GB. Is this easy to install? I'm new at this.

Since you have the 27" iMac it's very easy to install. In the back of the iMac is a RAM door for just that purpose. For running the apps you mentioned I'd get a couple of the 8 GB modules.

However, I'd only get the additional RAM modules from either Crucial.com or OWC (MacSales.com). They guarantee by testing the their modules will work in the Mac they are designed for.