Not sure how we got here in "TomTom Home and MyTomTom", so after answering, I'll move this one over to the Tech Support area.
I assume that what actually happened was that you "plugged it in to my computer to back it up" and Home pushed some new firmware your way -- new firmware for your ONE IQ was just recently released.
So let's start from the beginning and see how this goes.
FIRST and FOREMOST, it's important that we find valid maps on your PC or on your unit. So before doing anything, I want you to back your unit up - not using Home, but using this procedure:
https://www.tomtomforums.com/t17864-how-backup-contents-your-unit-computer.html
Once you have done that, it's time to go searching for your purchased maps on your unit so that we know you've got them safely tucked away before we go further. Please look in My Documents\TomTom\HOME\Downloads\complete\map. See what folders are there and there dates. Is there a folder there that has a date corresponding to the most recent map your purchased? If so, check that folder to be sure that at least the following files are present (and I'm going to guess it's a UK/ROI set - if not, substitute the area you had covered):
activation.zip
UK & ROI.toc
UK & ROI.zip
If those files are there, then we can go ahead and nuke your unit.
Why it's taking you 24 hours to format your unit is a bit of a puzzle. Even at the slow USB 1.1 speeds, it should take nowhere near so long to communicate with your device.
Connect your unit to your PC.
If Home fires up, just X it out or ignore it.
Format it again with your PC. Use FAT32, and do NOT use "Quick Format". When done, double check to see that you have something just under 2GB available to you on your device.
NOW start talking to Home. Use the "Update" function to cause Home to send you new code. The current code for your unit will be 9.400. If an earlier version is offered, go ahead and accept and install that, disconnect your device properly when done, and repeat the process until 9.400 is offered and successfully installed.
Now that you have a unit with good firmware, find the latest map from the folder we identified earlier. Home SHOULD know about it if you review what's on your PC with the Manage function of Home. If Home can see that map, go ahead and use Home to install it.
If anything breaks in the chain of events above, stop there and let us know what happened, but please do not proceed further without discussion.