When you return we will need you to completely shutdown the computer for ten minutes or so and then restart it using the power button, when you do this can you time how long it takes to get to the Windows desktop from the moment that you press the power on button, you can use the second hand on a watch for this.
The reason why accessing the data drive is so slow is because of the low amount of free storage space on it, this was mentioned back in my reply #6 but you do not appear to have taken onboard the danger of the free storage space being so low, the below is my canned warning for situations like yours;
For Windows to be able to run efficiently and to be able to update, on a mechanical HDD you need to have between 20 and 25% of the partition or drive available as free storage space at all times and on an SSD between 10 and 20%, if you don`t you risk Windows becoming corrupt or not being able to update which also puts you at risk of malware attack.
Data only storage devices should not be allowed to get any lower than 10% of free storage space of the full capacity of the drive/partition on the drive, this also to avoid data corruption.
Please note that storage devices can physically fail if the amount of free storage space is allowed to drop below the required 10 or 20/25% minimum.
Uninstall as many unused programs, games, videos and music files as you can and get yourself another means of backing up to asap.
Will catch up next weekend some time and you are welcome