500gb hard drive says its only 294gb with 293gb available.

We ( my husband and I) have an xbox 360 slim (factory settings never been touched by hackers) I bought a brand new 500gb hard drive for it. When i first put it in it said it had 460gb available, but after we started coping stored info over about half way through transferring info from 320gb drive with 295gb available I noticed the info for the new drive was off. We put everything back to the 320gb drive and the new 500gb drive said it was only 294gb drive with 293gb available.
We tried to clear the catch of the now empty drive and reformatted it but noting changes. When we first put the drive in it did say for few seconds 460gb then idk what happened.
I have tried to contact the company we bought it from and have yet to hear back from them.
I have no idea what to do. i do need help can this drive be fixed? I think they sent a lemon, but what if the drive was okay till we tried to use it. what caused the memory to reset to lower than 320gb drive. the 320gb drive we have has usable space of 295gb that is more than this 500gb. Its just not right.
Please dumb down help info pretend I am an air head. lol
some of the post i read in here nearly blew my mind with to much technical info I don't understand.

 

I had the same problem before with my 4GB Slim Xbox. It said only 400 MB were available, when it was a 4 GB. Then I formatted it, and then that's when problems became worse. I moved my profile to it, then after shutting my Xbox on and off it got PROD. After getting PROD from moving my profile so many times, it stopped finally and it said I have 3.0 GB available. I'm sure that this happens a lot, and I still don't even have my full 4 GB. Try this, move your profile to the HDD then turn off the Xbox, now turn it back on, if you get a red dot in the middle and the fans run really fast this is supposedly fixing it. If it doesn't work, then repeat this process until it does.

 

I did try this out of pure curiosity but it did not work. I never did initiate the RROD. I am guessing its what you meant by PROD (never heard of it)
I really don't want to mess up the whole system by trying to get a RROD

 

Did you have to enter the serial number of the console to format?
You can also try using this app on the xbox 360 called HDD Eraser, go to apps > browse apps > social > see all > and select hdd eraser.

 

I did have to enter the serial number for the console. I called Microsoft about it. They said the 500gb drives were made for the xbox 1 and the xbox 360 does not support the drive capacity without hacking the system. I would never hack the system.
\The weirdest part i really don't understand is it was advertised for the xbox 360 slim. but the geniuses working for xbox/microsoft said its not possible to have one.
i will try and find this app and see what it says.
it started when I was taking info from older320gb drive to the 500gb drive. it did at one point say 460gb free. Then with every item it copied to took it seemed way more mem than it should have. so we put the stuff back to the old drive and were left with less mem than we have with the the old drive. its just odd.
My worry is after all this, it will happen again. Microsoft said the xbox 360 slim is not made to have that big of hard drive. I would have to hack the system to get it to work and I just will not hack it. I would totally break it.