I'm missing 300GB of storage, why

officially, when you turn the system on the first time you'll have 802 GB of space left after the system areas are reserved, this size changes every time there's an update, but very likely that's where your space has gone

From
scripthacks.wordpress.com/.../calculate-usable-hard-drive-size

When a 300GB hard drive is sold the manufacturer is stating that the hard drive has a capacity which meets the following equation:

300GB = 300,000,000,000 bytes

With a 300GB hard drive the manufacturer would like you to think the above equation is true. It in fact is not. The reason why this value is false is because 1,000 bytes should be represented as 1,024 bytes. So in order to find the true size of a 300GB
hard drive you must divide by 1024 three times to calculate its true size in Gigabytes.

300,000,000,000 bytes ÷ 1024 = 292,968,750 Kilobytes

292,968,750 kilobytes ÷ 1024 = 286,102.3 Megabytes

286,102.3 Megbytes ÷ 1024 = 279.3968 Gigabytes

The corrected size of an advertised 300GB drive is actually 279GB and change.

If hard drive manufacturers gave you the advertised capacity of 300GB the drive would have to be 322,122,547,200 Bytes in size. This can be calculated by multiplying 300GB x 1024 three times to find the required bytes.

Personally I'm not a great fan of SSDs at the moment. I have my OS on a 120gb OCZ Vertex Plus and to be honest once started up I'm struggling to see the performance benefit compared to when I had the OS on a Seagate 1tb SSHD.

For me the performance benefits of SSDs do not compensate the higher price for less storage space.

There is no 300gb hard drive for the 360s,only 250gb or 320gb, so you have not got an official hdd.That'll be why you can't play the game.

It's not the classics edition and its the original game. The hd I got off amazon, I doubt it was made by Microsoft because it was 300gb for 45 bucks, however when I open up my storage it says Microsoft next to the hd.

This does not affect speeds because it is only the RAM that affects the speed of the xbox. The hard drive is what your game storage is being saved on. RAM only stores data while the console is on. RDR2 will not be anywhere near the 300GB mark so you shouldn’t
need to worry.