Dell Precision 490: Revert to Windows 7?

You don't need a new drive if the drive is in good condition/has not failed--just a clean custom install of windows. A custom install will allow you to delete the partition with windows 10 and install win 7 on the empty unallocated partition. Windows will format and install win 7. All files and programs must be reinstalled from backups or copies.You do need a dell installation disk with the same version as your license that came on the computer when new and your win 7 license key. 

Try shutting down (not Restart) reboot and start tapping F12 without waiting for any screen to access the boot menu.* Run the Diagnostics with a custom test of the hard drive to check the drive. *Check your manual for specific instructions on accessing the boot menu. It might be different on your model.

Actually windows 7 will be unable to remove the windows 10 partitions.

You will have to use diskpart with windows PE and remove them that way. The partition is protected in such a way that even the powerful Disk Management tool cannot do anything to it. Note that it's not because the partition is EFI, it's because the tool that created that partition had marked it in a way that prohibits other tools to tamper with it. (That's usually the case for the system hard disks formatted on with windows 10.) However, what if we want to delete such a partition and re-initialize the disk from scratch?

While the Disk Management tool is helpless in this situation, fortunately Windows offers yet another tool, DISKPART, that can do things to the disks that Disk Management can't. The tricky part is, that DISKPART is a command-line tool, that requires us to type commands into its command prompt to make it do what we want.

To get access to the DISKPART tool, first let's open the Windows command prompt in the "administrator" mode. We can do that by clicking the Start button and entering cmd in the search box:

http://knowledge.seagate.com/articles/en_US/FAQ/005929en?language=en_US

Thank you. That sounds hellish. What if I removed the C Drive and installed a blank drive in its place - then loaded the Win7 disc, wouldn't that do the job please?

DBAN or whitecanyon wipedrive might also do the trick.

http://http://www.dban.org/

https://www.whitecanyon.com/consumer-wipedrive