I'm using the Spanish version of Windows, so the names I give you may not be really accurate. But let's try it anyway:
-Open Settings > Control Panel > Management tools > Computer Management > Disk Management
Once this is open you'll have a list with both hard disks and below a graphic representation of them.
- Right click on E:
- Change letter and access path to the unit
- Add...
- Mount in following NTFS empty folder
- Create a folder inside C: (for example C:\E)
- Select it
- Press OK.
From now on you'll be able to use that folder as a normal folder although you are actually using E:.
This is not the solution I told you (actually merging both disks as if they were one) But it approaches. I can't seem to find that tool...
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-hope it helped anyway.