AOMEI Partition Assistant Standard is a FREE SD card partition tool for Windows PC users. It has a variety of features apart from SD card partition. For example, it allows you to migrate OS from HDD to SSD without reinstalling, convert disk between MBR and GPT without data loss, create Windows 10 To Go USB, convert NTFS to FAT32 and so on.

Before partitioning SD card:

Connect SD Card to computer and make sure it can be detected by Windows. Some laptop and desktop computers have built-in memory card slots. Otherwise, you may need a USB card reader to connect SD card to your computer.
It is a good habit to backup important data before making any change on your SD card. You can protect your files with free AOMEI Backupper Standard.
Free Download AOMEI Partition Assistant. It supports all versions of Windows 10, Windows 8.1/8, Windows 7, Vista and XP.
Usually, a used SD card already has some partitions. In the following example, I will divide my 32GB SD card, which was originally partitioned as a single FAT32 partition, into two drives.

Step 1: Install and launch AOMEI Partition Assistant. All the storage devices will be displayed. A SD card will be labeled as "Removable". Right click on the existing partition (here is G: drive) and select "Resize Partition".

Resize SD Card

Step 2: In the pop-up window, position your mouse pointer on the right border of the partition and drag the slider bar towards the left side to adjust the partition size. Then, click "OK".

Shrink Partition

Tips: To continue the next step and keep data safe, you need to click "Apply" on the toolbar to commit the previous pending operations first.

Step 3: You will find there is a block of unallocated space on your SD card. Right click on it and select "Create Partition".

Create Second Partition

Step 4: In the pop-up window, click the "File System" to choose "EXT2" or "EXT3", and then click "Advanced" if you want to give a name (here is Ext3) in the "Partition Label" box. At last, click "OK" to continue.

Ext3 Partition

Step 5: You can preview the new created Ext3 partition, but the change will not take effect until you click "Apply" button on top left corner.

Click Apply

Tips: On removable media devices, only the first partition can be assigned a drive letter. A partition without drive letter will be hidden from Windows Explorer.

Partitioning SD card really is as simple as that. There is no need to format your SD card. The Ext3 partition is going to increase internal memory on your Android phone.