There are several reasons why a PC will report that the Operating System or Hard Drive cannot be detected on start up. Some of these issues can be resolved without replacing parts and some will require you to narrow down the faulty part before taking this further.
If there are any other issues with the machine, please troubleshoot those faults before following this guide.
The first question would normally be what type of error are you getting when the PC starts up? There are several it could be, I've listed them below. However the initial troubleshooting for any Hard Drive is going to be identical. If there is any damage to the system it will not be considered to be In Warranty, unless you have Accidental Damage Cover in addition to your standard warranty. If there is no damage then please proceed to the first question in the next section.
Hard Drive not Detected No Boot Device Found No Boot Partition Found Data error Reading Drive Seek error - Sector not found Missing Operating System Drive not Ready Blank screen with flashing Cursor Hard Disk Drive Failure Primary Hard Disk Failure
If possible two way swap the Hard Drive with the Hard Drive from another identical system. Does the issue follow the original Hard Drive to the known good working system or does the issue stay with the original machine with the known good working Hard Drive installed?
If it isn't possible to swap the Hard Drive between machines and your system only has one Hard Drive, then it will depend on whether your system is a desktop or notebook for the next step.
If it is a desktop system I would recommend swapping the power and data cables from the back of the Optical Drive to the back of your Hard Drive and try booting once more. If the system boots then you will want to test the original power and data cable individually to determine if the fault is with a particular cable, a particular port on your Motherboard or with the Hard Drive. Once you have identified the faulty part go to the fifth step in the last section.
If it is a notebook system there will usually only be one connector or cable. If reseating the drive has not had any effect you will need to go to the fifth step in the last section.
Note: You can find a step by step guide to removing and reseating parts for your particular model or system in your User Guide. You can get this with your Service Tag at the Dell Support Site.
If the answer is YES, the system is still in warranty you will need to contact your Technical support to log a service call with your troubleshooting at this point.
If the answer is NO, your system is out of warranty you can still contact your Technical support for a quote for how much a chargeable repair will cost. Alternatively you can source a Hard Drive and fit the part yourself.
A general guide to Hard Drives can be found on the link below. It includes Instructions on how to view your manual online and has links to other articles on further Hard Drive issues if required. If none of these guides resolves your issue then please contact your Technical Support for further help.
Note: A Dell external HDD comes with a standard 1 year warranty and does not carry the warranty of a Dell PC. Most external HDD's are manufactured by third party companies and the warranty for the device remains with that particular company. If the fault is confirmed as being with a third party drive, you will need to contact the manufacturer.