I have an external hdd and i want to partion it into 2 partitions which are ext4 and ntfs. Is that OK with this kind of partitioning and Is there a problem with partitioning an external hdd rather than internal one? Thank you!
Assuming you are using Linux (and maybe Windows).
Using Linux, you can mount both kinds of partitions.
Using Windows, you will be able to mount the NTFS partition. The ext4 will require an external tool.
Mounting the NTFS partition on Windows from an external drive will not mess up the ext4, as long as you don't force Windows to do so (like formating...).
Partitioning an external hard drive is no different from partitioning an internal hard drive.
So, yes, having ext4 and ntfs in different partitions on the same disk is fine.