Hello there,
I installed Linux Mint Cinnamon 20 as a single OS for my laptop, erasing Windows 7. But I forgot to partition the disk. Now I want to keep the OS data and user data on different partitions. What may be done now? May I partition now without disturbance to OS files? How? Or I have to go through re-installation?

Please help me.

Regards

you need to boot another OS (from pendrive?) shrink partition, add new partition and reconfigure your system.
Probably faster to reinstall with different partitioning, if there was no any [important] data stored on that disk. Anyway, better to save your files before start.

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Years ago, I did this (moving /home to a separate partition) -it can be done fairly easily, but rather nerve-wracking for a newbie!

Unlike Windows, configuration files do not refer to files by a disk identifier, so you will not get any problems because /home is mounted as a different partition.

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Yes, you are right. It is very confusing for me at the time. Actually I can't figure out what is mounting in Linux terminology? what is mounting a directory? Like when you say;

To linux everything is one big file system (tree). The OS presents everything to the user as part of that tree. So to access the data on a device, for example a file partition, it has to be attached to, i.e. mounted on, that tree. The mount point is an existing directory. If you look at the /dev directory you will see lots of devices, some of which don't exist but that is another story. But the ones you will be working with will be sda1, sda2 or something like that. sda is the first hard disk, sda1 is the first partition on the first hard disk. I'm not sure what an sdd is listed as but the nomenclature will follow a similar pattern. You may also see usb1, etc., i.e. usb devices. So you mount devices with the mount command, e.g. That means attach device sdb2, the second partition on the second hard drive, at the directory /home/user/tmp. From then on you don't worry about the device, just use the directory.

Now directly to your problem. You can not make changes to a mounted device, in your case partition. Since you only have one partition, the running system, you cannot change it while it is running. You can reinstall setting up the partitions as you want them, or you can change things. To change them you cannot unmount your system or it will freeze, no system to run. So what you do is run another system, for example the system on an installation disk, and repartition your drive.

I would recomment using gparted. It should be available, perhaps as an icon or in the menu or else from the terminal with Reduce the size of your single partition and create a new partition in the now unallocated space. DO NOT MOVE THE EXISTING PARTITION, just shrink it or GRUB won't be able to find it and you can't boot. Then you will have to reinstall unless you know how to install GRUB. But now you have the second partition you wanted, so you can reboot, manually mount that second partition, and off you go. Of course you will probably want to set it up for automatic mounting in /etc/fstab but that is another story. See for example.

By the way, your home directory should /home/<username>. That is where you will mount the new partition. Be aware that you will probablywant to mount it elsewhere first, copy the existing home directory to the new one, and then mount the new one at /home/<username>.

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If you don't mind losing any data, a custom reinstall is quicker/safer.

Of course, as long as yo have backups you can retain backed up data.
Also, if you attempt an in-place modification and it goes bad, a full reinstall with partitioning during the install is still a fallback option.

Let fear make you take precautions, do research and hesitate long enough to consider the risk: never let it stop you.

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Thank you very much all.

Thank you @agillator for your brief on I have, however, opted a re-installation.

I'm not surprised. In most cases that is the easiest and safest option. At least you know you have options, now, and hopefully understand the filesystem a little better. Glad I could help.