Can I plug an old HDD (EXT4 partitions) in a new EXSi server and load all the existing virtual machines from there?
Is this is even possible? (mount my old EXT4 partition inside ESXi 6.5 and load virtual machines from there). Or is it really necesary to format my old HDD in a VMFS file system?
I'm totally new to ESXi, My company used to virtualize in VMWare Workstation for Linux, but today they asked me to migrate our VMs to a new ESXi server.
In answer to your question, as described: no, you cannot plug a EXT4 formatted hard drive into ESXi and expect it to work. This is because ESXi can only read VMFS formatted hard drives.
You should accomplish your goal by inserting the hard drive into an existing Windows server, (or even a workstation or laptop for that matter), downloading and running vCenter Converter, and converting/transferring all your VMs to the ESXi server.
ESXi is extremely limited when it comes to handling file systems -- so you can't just plug in a disk and expect to use it (not even a common-formatted USB disk).
I think the simplest solution is to have a target datastore on a drive formatted by ESXi and then copy VMs from a Linux machine over the network.
It is easy to do this with SSH -- you just have to enable SSH access to the ESXi server.
As of ESXi 6.7.0, look for Host->Manage->Services->TMS-SSH.
Now, from Linux, you can use scp, sftp or rsync (or any other SSH client) to transfer files and manipulate the file system that is managed by ESXi.
This solution can work with a Windows client also (if the disk has a Windows-compatible file system), as mentioned in some the answers here.
The question of VM compatibility is another matter.
