hello frnds i had already attached 4 devices in my computer through SATA cable i.e. three hdd and one CD rom, now i want to attached one more sata hdd without removing cdrom and using sata casing which is use to convert sata HDD to extenal hdd. kindly help me or let me know is there any cable available in market by which we can use one sata port for two hdd.
I suggest you look for these three options, in order:
1. Although your mobo has 4 SATA ports all in use now, does it also have one or two MORE SATA ports operated by a second SATA controller chip? You could connect any SATA device to one of those ports, although you probably could not use that device to BOOT from. If you do this, I expect you will have to Enable this new SATA port in BIOS setup, and Windows will need to load a device driver for this port and its controller - it may do that for you automatically after you have Enabled the port and connected a SATA device to it.
2. A similar suggestion: does your mobo also include an eSATA port, with a connector for it on the mobo and a small cable to connect to an eSATA port on the outside of the case at the back? If so, you probably would have to Enable this port in BIOS Setup, and have Windows install the device driver for this new port, similar to above. Once that it done, you could then connect an external drive unit that is designed to use the eSATA port. Note that this is NOT exactly the same as using an existing mobo SATA port and an adapter plate to create an external connector for an eSATA device - this latter process still uses up a "normal" mobo SATA port you don't have available.
3. As pdxalex suggested, you could buy a simple SATA controller card for a PCIe slot, giving you more internal SATA ports to use. This probably will involve Windows installing a device driver, too, but that is likely automatic and would be explained in the instructions for the card. Alternatively, if you want the new drive to be external, you could get an eSATA port card instead.
If you are considering making your fourth drive an external unit, it does NOT have to be eSATA, although I assumed above that it might be. When you get or assemble an external drive, there are two interfaces to consider. One is the interface inside the enclosure - how you connect the actual HDD unit to the inside of the enclosure. Usually these days it is for a SATA drive, 3½" form factor. The other is the interface between the enclosure and the computer. This could be eSATA, which is pretty fast - about as fast as using an internal SATA HDD. Or it could be USB3 - also just as fast. Or it could be USB2 - very common and widely used, but slower than eSATA. In fact, may enclosures and many complete external drive units come with more than one interface available, and you use only one. For example, mine has both eSATA and USB2, and I use the eSATA connection system because my mobo has an eSATA port built in.