How do I resize Linux Mint 21.3 partition on late 2013 iMac

You will need to delete the Linux partition & resize the macOS partition again, followed by reinstalling Linux. This is why I never recommend partitioning any drive. Or if you don't need hardware GPU acceleration or full system resources of bare metal, then installing Linux into a Virtual Machine is a good option since it saves on having to reboot to access the other OS.

It would be better & less risky to install Linux onto an external USB3 SSD. Besides that original Apple internal hard drive is likely wearing out or possibly even failing after all these years. The internal hard drive is definitely going to be slow even if it is still healthy. I guess you could of course install macOS on an external USB3 SSD and install Linux onto the internal HD. It all depends on how you intend to use each OS which way may be better.

On the other hand, if the Mac is otherwise working well - if a 1TB SATA HDD is installed, they could also get a 2TB SATA SSD pretty inexpensively, clone their macOS over to that and then re-partition that drive and clone the existing Linux install from the HDD to the SSD Linux partition.

Total investment of 1 x 2TB SSD, a cheap drive dock, and the time/effort required for the swap and cloning.

g_wolfman wrote:

On the other hand, if the Mac is otherwise working well - if a 1TB SATA HDD is installed, they could also get a 2TB SATA SSD pretty inexpensively, clone their macOS over to that and then re-partition that drive and clone the existing Linux install from the HDD to the SSD Linux partition.

Total investment of 1 x 2TB SSD, a cheap drive dock, and the time/effort required for the swap and cloning.

I don't believe the OP has the technical knowledge to do this (no disrespect to the OP, it takes time to learn these things)....especially the Linux portion. Unless the utilities have changed since I last used them, it usually took some manual intervention to clone a Linux setup since it usually requires updating the bootloader to work on the new drive & the changed layout.

I had thought about suggesting it, but keeping one OS on a drive is the simplest solution & easiest to maintain even for experienced users (or using a VM). Of course if the OP is interested in researching & experimenting it definitely would be a learning experience, but most people don't like pursuing those types of puzzles or the time it actually takes to do it.

Valid, although if someone is playing with a dual-boot Mac/Linux configuration, I tend to think they have a bit more interest in technical challenges than average. But of course it is wrong to just assume that.

g_wolfman wrote:

Valid, although if someone is playing with a dual-boot Mac/Linux configuration, I tend to think they have a bit more interest in technical challenges than average. But of course it is wrong to just assume that.

Not on this forum, at least from what I've seen. Linux is just another one of those Internet curiosities for many. There have been a couple of people who seemed at first to have a bit more technical expertise & interest, but when I tried to assist them it quickly became apparent they either did not have sufficient proficiency or sufficient interest in pursuing it. I know from my own personal experiences of cloning non-macOS boot drives, it does require the user to do a lot of research and sometimes experimentation. Plus Linux usually requires more use of the command line when performing troubleshooting & initiating fixes. I can only provide general guidance, the user would need to figure out the actual details necessary for their system.

Anyone hoping to learn or experiment with Linux should either install Linux into a VM, or onto an external drive, or onto a spare system.....at least until they become comfortable & familiar enough with Linux to use it on their main system. Even when people are comfortable with Linux, I still don't recommend partitioning or sharing any drive (my own personal experience and seeing all the problems people have on this forum when they do so including Windows Bootcamp).

Guess I'm just still an optimist - I mean, it won't last...