Resizing (enlarging) existing internal partition

I seem to remember that it is not possible to change a partition's size after it is made. What you would have to do is to backup your data, then erase the drive and repartition it. (Otherwise, the size change might overwrite existing data.) Or you could do what I've done. I installed an external solid state (SSD) drive and install Catalina on it. Depending on whether your computer has an older rotating hard drive, this will also greatly speed up the computer when operating on Catalina. These USB drives are reasonably priced and come in many sizes.

+1 to what Ronasara has shared. It was easier to resize partitions before the APFS format, but even then the second partition was always in jeopardy. I also agree that it would be easier and less risky to get or build an external USB 3.0 SSD, format and install Catalina on it and then migrate the data on Catalina partition to it. Then with a good Mavericks backup, erase the Catalina partition and attempt to add that space back to the Mavericks partition. You could also do the opposite and run Mavericks on the external SSD, but that would require more juggling and create a greater risk of losing your current Mavericks functionality.

Not to confuse your dilemma even more, but if that is a 21.5" iMac with the slow 5400rpm HDD. I would be inclined to setup two USB 3.0 external SSD's, one for Mavericks, one for Catalina and use the slow internal HDD for storage or backup.

It is not a good idea to partition the drive, what you should have done is to install Catalina on an external SSD. This is FAR more elegant a solution than partitioning, particularly when you don't know what you are doing.

kris10dee wrote:

thank you! Not sure which what the HDD speed is on this Mac - it's late 2012 and I've never felt it particularly slow for my needs...but I'll check it!

You're welcome.

The Late 2012 27" iMac's came with a 3.5" 7200rpm HDD and the speed is around 100MB/s.

An external USB 3.0 SSD has a speed closer to around 400MB/s and you would notice a big difference.

+1