When you boot up from your boot C: drive, is that a SATA HDD and in the BIOS to you have the SATA config set to AHCI? Might check.
I am assumming you can see the ext IDE drive fine, can see files and folders properly, so that we know the USB/IDE bridge is OK.
This could be due to a boot virus, however the error message can occur if you have the SATA mode set to AHCI, when you are trying to boot with an IDE drive. ("STOP 0x0000007B INACCESSABLE_BOOT_DEVICE").
If so, you would have to decide how badly you want to boot from the ext IDE drive, and if so, could consider making a change in the registry to load the IDE boot drivers, and changing the SATA config back to IDE.
Yes, that's the error message you get when you have the boot drivers set for AHCI, and you try to boot with an IDE HDD. You get the same thing if you initially install windows on your computer in IDE mode, then change the SATA configuration to AHCI.
You "can" make a change in the registry, for example, to set the PCIDE drivers to load with the boot drivers. Then you have to go back in the BIOS and change the SATA config from AHIC to IDE. And then your SATA SSD would function in the old IDE mode.
I don't know anyone that has gone from the latest AHCI to the older IDE mode. Don't know your Dell BIOS, or even if you can run in IDE mode.
Probably the best solution is to use your older IDE drive as a secondary drive which will work just fine, and not change your SSD AHCI configuration. If you absolutely want to dual boot with a different OS, consider a new SATA HDD or a different SSD you could place the alternate OS on.
By secondary drive, I mean a non boot drive or partition. Use the IDE/USB drive for storage, that doesn't have to be fast.
I don't know how Dell set up their BIOS and what key they use to set the AHCI drivers to boot initially, they may use another key AIStor. Might check in the BIOS to see if IDE or AHCI mode is selected. IDE wasn't listed as a choice from the info in your manual.
It used to be by default drives were set in IDE mode where the IDE drivers were loaded initially, but now, like your Dell manual says, many are setting their configuration to AHCI as the default to integrate the advantages of it. The new ASUS boards are that way also.
I would not change the registry to try to get your system to boot from the USB/IDE drive. You have to make a registry change, then shut down and reboot, then make the BIOS change. If there is a problem, or it's not done exactly, you can't boot, and therefore you can't get back into the registry to correct the problem. AHCI is the better mode and you don't want to break something that isn't presently broken.