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After downloading inventor, I tried instaling it and even though I specify the program to go to the D drive it defaults to the C drive which does not have enough space.
Regardless of the drive, there are going to be files that must be placed on C:\ such as prerequisites and files within your Windows profile. I suggest you clear off some data on your C:\ and move it to D:\ so that you can complete your installation.
For performace reasons, you should never have less than 20% free on your C:\ drive.