Partition keeps dissapearing

Hi! Im kindly asking for your help please ladies and gentledudes.I have an Acer Aspire 8920G with 500gb dual HDD(250x2) running Windows Vista Home Premium.Local disk(D)keeps disappearing and only shows up again when i restart the PC.

Firstly i would like to find out if its actually a physically separate disk on the laptop or part of a partition.I know it says Dual HDD but I am a bit confused and just need that cleared up.please

Secondly i would like help to stop it playing the disappearing act or any information on what may possibly be causing this.

Lastly i would like to optimize performance,security etc.If someone could point me where i can get a guide on how to partition my disk or disks to get the best out of my PC.Here are the current partitions below

Disk 0
-10GB Eisa Configuration(what`s this?)
-3.54 Eisa Configuration(what`s this?)
-111.44gb Acer(C)NTFS
-107.90gb Data(E)NTFS

Disk 1
-232.88gb Local Disk (D)

Your help will be greatly appreciated.I am not that good with computers so if u could keep it simple that will be nice.I`m kinda average i think