As with real PCs, virtual machines in VirtualBox can emulate the three types of disk controllers available today: IDE, SATA, and SCSI. The default type is IDE whenever you create a new virtual machine. And why is that? Well, the main reason is that almost every operating system available nowadays can recognize IDE devices. But some operating systems such as Linux, OpenSolaris, and Windows Vista can also work with SCSI and SATA drives. SCSI disks are more commonly used on big data/application servers that require the use of RAID (Redundant Array of Independent Disks) technology to be fault-tolerant, although you can also use SATA disks for that kind of environment.
My advice for you is to use the default IDE controller type for most virtual machines unless you need the fastest configuration available, in which case I'd recommend using SATA drives. And if you create a new hard drive using the IDE controller but later you want to use the...