Welcome to Android Central! It's important to know that if you have a clone phone, then to a certain extent, all bets are off--you just can't be sure how reliable the software or the hardware are going to be, so your problems with external SD card access and PC connection could be an inherent glitch in the device.
As for SD cards, it's important to distinguish between the external SD card (the actual card you can plug in and remove) and the "internal SD" card, which is the device's onboard memory. The "internal SD" is also called Internal Storage, and is where all of your apps are installed (as well as your OS files). Most decent devices will come with at least 8 GB of Internal Storage, and the flagship devices come with at least 16 GB. Cheap devices often only have 4 GB of Internal Storage, and of that storage space, 2-3 GB is often taken up by the OS, leaving you only with about 1 GB of space to install apps and store things like text messages. You can't install apps directly to an external SD card, so it doesn't matter if you have some huge SD card installed.
So in your situation, can you clarify that it's the external SD card that is telling you there isn't enough space, or is it the the internal storage? It'd be very helpful if you could post a screenshot of your Storage stats from Settings>Storage.