My system has 4 disk drives: 128GB SSD boot drive, 500 GB HDD data drive, 1 TB HDD data drive, and a 64GB SSD work/temp drive. This is how it looks in Win7:

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X: is the SSD where I have placed My Documents and Temp files.

To boot Wini10 I replaced my C: drive with a new 128GB SSD and did a clean install of Win10 on it. I had 2 immediate problems: I could not activate Win10, and Win10 did not see 2 of my disk drives. With the help of someone here I was able to get my Win10 activated, but the system still will not see 2 of my drives. Here is what Win10 looks like:

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As you can see, Win10 sees all 4 drives, but for some reason it says D: is not accessible, and it does not give Disk3 (what is my X: in Win7) a drive letter - so it is inaccessible too.

Needless to say all drives work fine under Win7, so it is not clear to me what the problem might be.

Another peculiar issue is that when I switch boot drives and go back to booting from Win7 the system goes through a complete CHKDSK on all 3 of my non-boot drives. This sugggests that WIn10 is doing something to the drives that Win7 does not like. I'm a bit reluctant to keep switching between 7 & 10 because I'm concerned that I might lose a drive if the CHKDSK function fouls up or has some sort of problem.