Is your other Vault - the PC - online? Looking at the image you sent, your new hard drive Vault (called Mac Mini) already has synced all the thumbnail versions of your files, but it still needs many of the Preview and Original versions. Those would only exist in the other Vault, so in order for them to sync that PC/Vault has to be online. Also since the files are on a computer that is not connected directly to the new Drive/Vault, the sync is happening over the network, which is inherently slower than a direct connection.
Assuming your “Mac Mini” drive is formatted as ExFAT, you should consider connecting it directly to your PC and letting it sync completely, then move it back to to the computer called Frodes Mac Mini. Syncing of any new data will go much faster after that initial sync.
As for your other question, yes the Mac Mini computer should be set up as a separate device in order for you to use it and have any Vault attached to it be part of your Mylio ecosystem.
There’s another scenario I’ll mention in case that’s what you’re trying to accomplish. If your goal is to have the Mac Mini act as a vault, but for all the data to reside on the external drive (rather than the Mac Mini’s internal small drive), you can accomplish that by relocating the main Mylio folder for that device to your external drive. Some WARNINGS about this type of setup:
1 - In order for Mylio to work on your Mac Mini the external drive will always need to be connected and mounted.
2 - The drive itself should not be used as a Vault anymore, it will really just be treated as part of the Mac Mini computer. So the drive will need to be removed as a device in Mylio (Troubleshooting: Re-provision an External Drive as a Mylio Device - Mylio Support)
3 - Once the folder has been successfully migrated (Troubleshooting: Moving the Location of Your Mylio Library - Mylio Support) you’ll need to change the Mac Mini from Auto-Optimized to VAULT. This will force a sync all the originals from your other existing Vault (the PC).
All that is to say that this process does have some risks and will take a little time to accomplish since you’ll be copying your entire library again. So you should be very careful implementing it. But if your goal is to just have two devices - PC and MacMini - that both add as Vaults then this is how you can do it.
Please contact Support directly if you are not sure how to proceed or have questions about any of the above steps.