NEED HELP ASAP! Can't use Iomega Desktop Hard Drive

Several years ago I purchased an Iomega USB 2.0 Desktop Hard Drive (model LDHD-UP), at which time I had a Windows PC, so it was formatted using the NTFS default file system. I recently got it out to retrieve a document I needed, plugged it into my Macbook and all was fine. Since then I decided to try to clean it up because it has many files which are no longer needed. Unfortunately, for whatever reason, everything on it was "Read Only". So not only couldn't I edit anything without first copying to my Macbook, but I couldn't delete anything off it either or add anything from my Macbook.

As usual I attempted to fix the issue myself and about crashed Google in the process LOL I found an article on how to reformat so that it would, "work seamlessly with MAC and PC". I thought, awesome... this is exactly what I was looking for!

From there I plugged it into my old Windows laptop, which I had given to my dad, made a new folder on the Hard Drive, stuffed everything else into it and sent the folder to the desktop. After several hours of waiting for everything to transfer, I started the reformatting process. The article presented two formatting options, FAT and EXFAT, with pros and cons for each. I decided to go with EXFAT which it said can work with larger files, unlike FAT. So I started the reformatting process which literally took all night. The following morning it was all set to go, so I took the folder I had removed from it and sent it back to the Hard Drive... that's when the issues started!

Initially the files took up less than half the storage space on the Hard Drive (which I believe is 500GB), however, when I transferred them back it said I could not because there was not enough room, I need almost 900GB more! I thought WTF, how did the storage space shrink?! But when I view the property info for the Hard Drive, which is now empty, it shows a 465GB capacity. So I am really confused how files which came off of it initially took up less than 250GB, and now it's saying they about 1400GB.

So I started the tedious work of transferring files little by little until it was full. I then plugged it into my Macbook only to discover it no longer recognizes the device at all. Frankly whomever wrote the article I stupidly trusted is someone I'd like to drag into the street right about now!

But now I have no idea what to do. I tried to find support documents for the Hard Drive, but apparently Iomega is no longer around. Going to their website just gets me redirected to random websites. So I thought here might be my next best option.

First question I have... Is there anyway to actually format the Hard Drive so it can be used on both my dads Windows laptop and my Macbook?

Second question I have... Is it possible to restore a Hard Drive to factory settings, or otherwise get to how it was out of the box?

Thank you in advance to anyone who replies, and please, if you're going to suggest how to do something do it in as much detail and dumb it down for me as best you can. Obviously I am not tech savvy LOL