General Whs 2011 Question

Hello forum!

Hi y'all,
I am running WHS 2011 mainly for backup of 2 PCs and to host a fairly large media connection.
I am semi-proficient with computers. I was able to put HDs into an old desktop and get WHS running and backing up.
I backup the 2 PCs to an internal drive and BU the server to an external usb 3 HD.

I usually use the server for any adjustments as using it remotely is just not as easy.
So....just to let you know my user level.

My main question is what are all the 4GB files in Client BUs? There are many, comprising 223GB of data and that is just ONE day. The drive is 1TB and with the media in the serverfolders the drive is almost eaten up already.

Does WHS keep 1 or 2 client BUs and delete the old ones? I don't see a way to manage that.

edit: one more question. I know the server backup is working, or so the server says, so I know the usb 3 external drive is working...but I do not see it anywhere. It's not in disk manager and not visible in explorer. It is not a HD in the dashboard either. Has it been selected by WHS to be ONLY for server BUs?

Thanks for any help!
Chris