Startup disk full

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+2 - No. Adding Memory will not give you more storage. Instead, Free up storage space on your Mac - Apple Support.

You may also need to move large files and libraries on to a high-quality external hard drive, such as a SanDisk Professional G-DRIVE - Western Digital.

Jack

No. Memory is RAM, storage is your Disk; the hard drive or SSD. They are not the same.

To free up space on your drive, you delete user data. If it's important data, copy it to an external disk or a cloud storage service before deleting. iCloud is not designed to be cloud storage.

You would need to increase your hard drive storage, not memory. You can delete unused items, or move files to an external drive to save space. Assuming a 2012 iMac (as indicated), the replacement cost might be better served toward a new machine instead.