1) Usually it will NOT cause any sort of errors, but that depends on A) if your D and E drives have software from another OS which doesn’t support or are compatible with the new OS.
B) They’re formatted for Windows pre-installation, as long as you’ve been using these drives for Windows before installing your new OS it should work just fine.
2) In theory, yes you should be able to put that 15.6gb of unallocated space to your new C drive partition, if it doesn’t let you then try using Partition Wizard.
3) They should show IF they support Windows and/or if you’ve already had them formatted to be usable with Windows. For instance a Mac HDD won’t be accessible on a Windows OS without special software or a correct format like NTFS/FAT32.
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Via Windows Central