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so i have an android 9.0 phone i got a 32gb sd card for it for 8nzd and i use it as the internal storage as phone only has 4gb built in and and 400mb free space android uses the rest of the space

so i wanted to copy some files to my phone off my pc but when i go to this pc on my computer and click on my phone it only shows the internal 4gb storage no sd card

what can i do

While connected through USB, you might have an option in your notification (swipe down from the top on the phone) for how the connection to the PC is interfaced. Changing this dialog might make it show up.
Another option is to power the phone down, remove the card and plug it into the computer through a USB card reader. Most Oses recognise exFAT which is the format Android uses.
Still another option is to share folders from your PC through Samba (windows file sharing) on your network and acessing the files from your phone using one of the common file managers. I use Total Commander with the LAN Plugin addon, tho ES File Explorer is a more popular solution.