File Size Issues & Full HDDs?

[Vectraat]

Ok, I've got 4 questions.

1.) Can filling up a HDD to 100% capacity cause performance issues or shorten the lifespan of the hard drive? If so, what's the cutoff point before I'll notice performance degradation/shorten lifespan? Keeping in mind, I'm not running Windows/programs from this HDD, It's just used for storage/accessing media. Also, does the same rule apply to RAID? 

2.) I notice a few HDDs in my PC are only fastened down with 2 screws instead of 4 like the others in my computer. Due to the drive bay spacing of the screw holes, there's no way to use 4 screws. Since they're only held down by 2 screws I can easily lift up one end of the HDD with a single finger. I'm wondering, since they're not completely fastened with 4 screws, can this result is vibration issues over time? 

3.) I notice when I transfer files from one HDD to another, there can quite often be differences in bytes. Sometimes It's fairly dramatic and other times it's subtle. And yes, even with hidden files/folders selected and every other applicable folder option I can think of with just a single file in a folder, there can still be a difference. Even if I use TeraCopy to hash check via MD5 during and after the transfer, TeraCopy says the transferred file is 100% correct, if so, what would explain the reduced file size? I think someone said this may be a Windows bug.  

4.) If I right click a file > Properties, Windows says GB, but do they really mean GiB? Because I notice when I download files, websites will often say GiB, but when I check the file in Windows It'll say GB, but I think Windows really calculates in GiB, so why not list it as GiB? There's a difference.  

1 Gigabyte =  1000 Megabytes
1 Gibibyte = 1073.74 Megabytes

Also, kind've annoying, but if say--as an example--I download a file from a site that says: 26.68 GiB, then I right click > Properties in Windows, it'll just say 26.6x (missing the 8) which leaves me to calculate the bytes. WHY can't Windows display the full file size in GB/GiB? Is there a third party piece of software I can use that will make this calculation? Because I backup A LOT of data and It's very important for me to know if my transfers/backups are accurate. I need 100% identical files, but Windows 10 doesn't exactly instill confidence.