External HDD not readable

Formatted as what?

dlieber said:

Hi justperry,
Thank you for your quick reply. I formatted a first partition as Mac OS Extended (Journaled) and a second as NTFS.

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If it shows up in Disk Utilities there is a good chance it is repairable.
This is I think the first time I see someone using both NTFS and HFS + on an external drive, I myself would not do this due to compatibility.
This makes it complicated, you might succeed repairing the NTFS partition in Windows but what happens to the other HFS partition.
OOTB OS X does not have Read/Write Access and can't repair NTFS.
I would suggest you to download a trail of Paragon NTFS driver for Mac, it install a NTFS driver and repair tools, the repair tools are integrated with Disk Utility so try again after installing Paragon.
Another really great Software is DiskWarrior but there are 2 BUTS, one is it can only repair HFS+ and the other one is it is expensive and there is no trail.

dlieber said:

I was afraid that this might complicate things. I initially did these two partitions in order to host time machine backups on the HFS one and use the other one as a hard disk that could be shared with PC users. I would much rather have the NTFS partition recovered, if both can't be saved.

Anyway, I followed your advice to try the Paragon software but it didn't enable the verify or repair buttons in Disk Utility. One thing I should have said before is that the hard drive shows up in Disk Utility, but not its partitions. Moreover, everything is greyed out in each tab...

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Hmm, you see the HD but no partitions, think that's going to be hard to recover.
You might try to get the HD out of it's enclosure and try it in another one or in a PC, you never know, it might be the casing giving the problem although unlikely.