A disk error could be NUMEROUS things, not just a failed hard drive!!!! If you replace the hard drive and theres nothing wrong with it, you wasted your money and you lost all your data.
It could be that your bios is set to boot from another virtual disk. Check your bios. (this is unlikely because it failed at the pbr descriptor).
It could be your HD is fine and your MBR and partition table is jacked up from a virus or a crummy third part disk defrag program. PBR Descriptor 2 message is part of the normal boot process (directs the boot loader to Primary Boot Recond 2). Or the table points to the right place, but the boot record is corruupt. This is a good thing to check because MANY viruses attach the MBR and your system failed at loading pbr2. you might want to try a fixboot or a parallel install. Just google the terms and you will find instructions, it it too long to list here but EASY to do. Again, this is a common place for viruses to attack You could also try a fixmbr, but that might ruin the hhidden partitions.
It could be a virus ruined your os partition.
It could be just a few bad sectors in your partition, if so a scandisk or a chkdsk /r/f/p may help
It could be a bad mobo.
It could be a bad ESATA connector. or it might simply be misaligned. remove the screw slide your hd out then in again and PUT THE SCREW BACK IN. You can also try your drive in another computer or another drive in your computer.
It could be that the NTLOADER file is missing or corrupt. It could also be
NTDETECT.COM, BOOT.INI, BOOTSECT.DOS. If worse comes to worse, you can try a "parallel install" which would allow you to recover your data.
Your dell has a hardware dianostic partition on it. This will tell you for shure if it is a bad HD other bad hardware or simply a software issue.. Hit f12 at start up, choose diagnostics. Answer the questions and it will tell you if there is hardware problems. Ignore the optical drive part of the test. It tests reading and writing on a blank dvd drive and this is NOT part of your issue anyway so just skip or ignore that part.
Remember, dell has a hidden "return to factory image" partition if your HD is good and you backed up all your data. This is the only sure way to get rid of a virus.
The trouble with using both the hidden partitions is that if the pbr is jacked by a virus, it may prevent you from using them both. Watch what you download!