Storage drive disappears

First, right click computer, select manage and in the left column select Disk Management. Post a screenshot. Screenshots and Files - Upload and Post in Seven Forums

If you see the drive in the bottom section but it has no drive letter, right click it, select change drive letter or path, next screen select add, and give it a drive letter. If it wants to format it, do not do it. You will lose everything on the drive.
As to why it is happening, there could be several reasons. A loose cable could do it, or the drive could be dying. To determine if it is sata or and IDE drive, look inside your computer. An IDE drive will be connected with a wide ribon cable. Whichever type it is, make sure the cables are properly seated on both ends.

If you do get it back run checkdisk f/r Disk Check
Also, go to the manufacturer's web site and download and run their diagnostic tool.
The first picture is an IDE cable the second picture are a Sata cables.


sata power and data cables

Yes. Sounds like a dead/dying drive. Run the Manufacturer's diagnostic program. It will determine if there are problems. I would also, try to download any data on it you would like to save ASAP. If it survives the diagnostic test, run checkdisk using the f/r switch. If it can do it, it will take several hours probably. It is possible there are bad sectors on the drive, and the drive is salvageable. But, what you describe are indications of a dead or dying drive.

Did you test it? You may try to see if Seagate will RMA the drive. I've had good luck with seagate drives, others have not. Truthfully, I think it really does not matter. You're taking a chance no matter which brand you choose. The name of the game is backup, backup, backup. I have all my data on at least 3 different hard drives and backup to them several times a week, if not daily. I'm so paranoid, I use 1 drive for each computer that I clone my OS to weekly. If my OS drive went out, I'd be back online in 10 minutes, 1 if you have an eSATA dock.

I don't know how much you know about this so I'll try to see if this works with full details.
Click start. in the search box type cmd, on the cmd entry that appears, right click it and select run as administrator. (called an elevated command prompt)
in the box type diskpart and press enter, it will take a few seconds, but it will answer diskpart
type list disk and press enter - hopefully tour drive will show up with a number assigned to it. example 1
select disk 1 (or whatever number diskpart assigns it) it will reply disk 1 is the selected disk. Be sure you select the corect disk.
type clean all and press enter. go have lunch. it will take several hours, but the drive will be wiped completely. It will look like nothing is happening but it is. There is no progress account until it is finished. It should take 3-5 hours.
Assuming it is a data or storage drive. When it completes
(not really necessary, but to be sure) type select disk 1 again and press enter
type create partition primary press enter -response should be the selected partition was created
type select partition 1 press enter
format fs=ntfs quick and press enter
type assign and press enter. To assign it a drive letter
exit -enter to leave diskpart and go back to the command prompt
exit enter to leave the command prompt
If all that completed your drive should be ready to go. and should show up in disk management and explorer.

It should look similar to this but, type assign and not active unless you are going to install an operating system on it.