Cool, I'm happy to hear that you can start using that external drive now. :)

"something bad may happen to the new drive" -- The encryption is purely a software thing, there is no chance for physical damage, no matter how much you screw up. :) (Well, you could overheat the...

Well, if it's new and there's no data on it yet, you've got nothing to lose. Feel free to try Windows 7, my guess is that it would work just fine, but if it doesn't, you can just reformat /...

That's strange... So you booted from a recovery CD / pendrive, issued "manage-bde -resume" from the command line, and it's still as slow as from within the running OS?
If you want to go back to the...

TL;DR:
To get a 5x speed boost on encrypting your full drive in Windows 10:
1. Enable Bitlocker in the running OS, but pause the encryption once it's in progress
2. Create a recovery disk, boot...