Hi,
There are a few possibilities here.
First question would be; what was the HDD connected to originally?
For exemple, if it was used on a MAC the file system is different (Note : This does not work on Windows but, I have not tried on Linux, so only take this has a hint for further inspection if related).
Second, The disk could be corrupted/or in an unstable state. If it was used in Windows, I guess plugging it in a Windows machine will pop up a "repair" window to try and fix it.
Third, could the disk be encrypted/secured? If it was an internal laptop disk, it could be bitlocked by Windows or the computer's BIOS, etc.
Bonus : I have used different USB external casings in the past, some use a proprietary disk format and are not "readable" if you trnasfer them to another controller. Lacie disks and Western Disigtal MyBook's for example. Or, if it was in a RAID group from another computer/server or disk unit a.k.a. as a NAS drive.
Hope this can direct you to some solution.