Hard drive space is the amount of storage space you have on the drive. The memory card of a phone is similar but it's flash memory which stores data. They both store permanent data there. It's not RAM. (Random Access Memory.) If you don't have enough space on your phones internal memory storage card, then updates won't install because they need more space to install their files. Clean out some of the junk on the phones storage, such as pictures/photos and long text messages which contain photos. It will help. Likewise with your computers. Although some folks can use up all the HDD disk storage space, you have not. 75% available should never stop you from installing things.
On the other hand, in regards to a phone or a computer's RAM, a computer tends to use as much RAM as is available so it can temporarily store data and use it however the operating system wants to use it. If there's not enough RAM, the job will generally still get done, it just gets done slower. If you are having error messages regarding not enough RAM, then install more ( It's a cheap way to increase performance on a computer.) or reduce the number of programs or apps using that RAM. Because most folks leave all sorts of apps running, shut down unnecessary background programs and apps that steal RAM.
Hope this helps.
Grif,
seem to have enough ram? it seems as though and this is my 3rd computer over 25 years or more only. that my ram seems to suffer. I also have this problem on my phone. I don't have enough space on my phone to do updates. when I look at my computer hard drive 75% of it not being used.

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