Disk disappeared!

[MaxPol98A5]

For the second times in a few months, lightroom dumped on of my external drive (about 15000 images). In fact, the disk appear in the list, but none of the folders are there. There is also another drive in the list with a similar name, containing folders, but all offline (question mark). Also, folders are almost empty, containing only few images. I recently updated lightroom (as well as photoshop, camera raw... as well as windows). Why is this happening? I haven't moved or renamed folders on the disk; nothing moved.. Why is this happening? What can I do to avoid that? I'm now losing my time recconecting folders and files instead of working...

[AxelMatt]

Are you sure that the external drive is connected with the same name and drive letter as used on import?

If you access the drive with the Explorer you see all the folders and files? 

[johnrellis]

"There is also another drive in the list with a similar name, containing folders, but all offline (question mark)."

See this help article:

https://helpx.adobe.com/lightroom-classic/kb/lightroom-displays-same-hard-drive-twice.html 

[GoldingD]

There is also another drive in the list with a similar name, containing folders, but all offline (question mark)

I see your OS is Windows. Hard drives are given drive letters, the names associated are not important. LrC depends on the drive letters to not change. For those external hard drives, are the drive letters permanently assigned? 

Had your OS been MAC, then drive names would be important.

[GoldingD]

There is also another drive in the list with a similar name, containing folders, but all offline (question mark). 

Why offline? Are your external drives allowed to go into power save mode? It is important for external hard drives to not go asleep while running LrC, LrC will not constantly access those hard drives, so they could think it is time to conserve power. Prevent that.