Have a look in disk management (Start > Run > Type 'mmc' > File > Add/Remove Snap-in > Add > Select Disk Management and follow the prompts).
Does your disk show up here (it should, asthe Device Managem shows it)?
Is it partitioned and formatted (check the File System column)?
Also, has it been assigned a drive letter (check the volume column)?

If it is not partitioned/formatted you'll need to do that. I'd say that the problem is drive letter assignment though (either it doesn't have one, or there is a conflict), this can be remedied by right-clicking the partition on your second drive (below the column display) and selecting Change Drive Letter and Paths.

I've attached a screen shot of the Disk Management tool.
As you can see, it only shows the second hdd below the column list.

I think I might know why it's not listed correctly, but am unsure how to resolve it.

The reason I had to reinstall XP was because I defraged the first hdd when it had Acronis OS Selector on it. There is a bug in that software which happens when you defrag a drive. A possible solution to it was to fix the MBR (Master Boot Record). So, I downloaded UltimateBoot CD in order to fix it (no floppy drive anymore).

While going through the steps to fix the MBR, it asked which drive number to run it on. I entered the number 1. But because of stupid computer logic, I should have entered 0 (HDD 1 = Disk 0, HDD2 = Disk 1). Therefore, I ended up wiping the MBR for the second hdd.

Because I assumed that it didn't resolve the problem (was still getting the error), I decided to run my Computer Recovery CD's, which wiped my first hdd and reinstalled WinXP (losing all data!).

Am I right in thinking all this?

What can I do to fix the second hdd? I cannot afford to format it (there's too much valuable data on it).

I tried using the Partition Recovery tool that comes with 7Tools Partition Manager, but it didn't work (although I could be doing it wrong).

Your second hard disk is Disk 1.
It isn't partitioned or formatted. You can do this within Disk Management by:

1. Right click on the space listed as unallocated.
2. At the menu that comes up, click on the New Partition option.
3. You will now be presented with a wizard as to how you would like the partition to be created. At the first screen press Next.
4. At the following screen, determine if you need a primary or extended partition. For more information about these partitions you can read our partition tutorial listed above. Most people will be fine selecting Primary Partition. Select Primary Partition and press Next.
5. At the next screen you will be prompted to type in how much of the unallocated space you would like used for the new partition. In my example, my unallocated space is 26.43 GB, so I can choose to make a partition of that entire amount of only use half and save the rest for another partition. Regardless, of what you decide, you need to enter a size in the Partition size in MB: field, or stick with the default size, and press the Next button.
6. At the next screen select the drive letter you would like assigned to it, or use the default one given. When done, press the Next button. The drive letter that you assign here will be how you access the partition later.
7. You will now be asked which sort of file system you want the disk formatted as. Go with NTFS.
8. Type a name in the Volume Label field that will be associated with this partition or leave it blank.
9. Finally press the Next button and you will come to a summary screen. Review how the partition will be created, and if you are satisfied, press the Finish button to complete the creation and formatting of your new hard disk partition.

If I do this will I lose the info on the disk?

The disk isn't currently partitioned or formatted, so unfortunately you have probably already lost the data.
Even if you haven't, you can't access the drive now and repartitioning it will wipe the data.
Unless you use a commercial product like Partition Magic any partitionioning you do is destructive (IE wipes the data), if you can afford it you can use PM instead of Disk Management and might be able to get some of the data back.
Otherwise, I hope you make backups.

That's just it, the 2nd hdd was supposed to be for backups. I've learnt my lesson now and plan on purchasing proper backup software and will run daily backups between my two computers.

I've not given up on retrieving the data. I'm going to get Partition Magic and try to fix this.

Thanks for your help David

I'm also having issues with a second hdd used as storage. My OS (Windows Vista Ultimate, 64-bit) does not show the drive under "My Computer", but it does show up in the "Device Manager" as working properly. It also shows up in "Disk Management" as healthy (active, primary partition), though without a drive letter. When I go to assign it a new drive letter by right clicking on it, however, all the options are greyed out except for "Delete Volume" and "Help". I've tried using the diskpart command under DOS, but the only volumes listed are my primary hdd and my dvd+/-rw drive. The drive isn't listed as having an NTFS or FAT32 file system - does this mean it's no longer formatted?

My primary drive (C:\) is a SATA drive, and both the dvd drive and secondary hdd (the one in question) are on the same IDE cable - this second hdd is running as slave to the dvd drive. I recently upgraded computers (both hardware and OS - the old OS was WinXP SP2), and was having the same issue with this drive in the previous machine - partitioned healthy drive, but no drive letter.

Please help! I've gone for a few months without the data on this drive (if it's still there), and don't want to risk deleting the volume just to gain access to a second hdd!

hai
i think u r computer is reinstalled with win xp and u reconnected u r second hdd in it, but i hope u forgot to set the first hdd jumper settings as master and the second hdd as slave of first hdd,if even that the problem is nt over then chuck u r data cable and its pins

I think you get the point