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I just incompletely wiped out an external HDD

Was installing Windows 10. Cleared the C:/ partition on my internal hard drive, only then to be greeted by the message that my HDD uses MBR and not GPT (don’t know if I’m getting that right).
So I realised, I obviously have to clear the entire HDD now, cause I have already deleted the C:/drive containing the OS and all and changing MBR to GPT via a software is now a long gone option.
Good thing I backed up important files on one partition of my internal HDD. So I think of copying all of them to an external HDD. Since I somehow want this to be “tip-top”, I decide to WIPE OUT my external HDD before transferring files to it.
The wipe out begins, but I grow impatient and stop it around 0.75% or so.
This is where the problem occurs. I plug my ext-HDD out and plug it in again. Noting shows up. No I:/ or anything, no NTFS, nothing. Only a “Drive 2” is visible in the wipe out section.

What should I do? What happens if I wipe it out totally? Will I have the chance of “allocating” it NTFS or something once the wipe out is done?

Also, is there a faster solution to this entire thing? Like a software that can run through the setup screen itself and concert MBR partitions to GPT? Is there a way to keep my files intact without the ext-HDD atleast?