The external drive probably got a new drive letter:
2 solutions possible:
1)Find the letter (or full path) to the Library on that drive.
In Calibre: Right-click the Library Icon: Create-Switch: USE EXISTING: <navigate to where you found it>. That should do it.
2) WINDOWS Systems Lock the drive letter
Start
Disk Management: right click in the (external Library) drives active partition: change drive letter: Pick a higher letter that will not normally ever be needed when attaching other devices. L for Library is a good choice
Now that drive will always get the same letter when attached to that machine. (I think it may get the letter when attached to other Windows systems, but don't count on it)
for more info
https://docs.microsoft.com/en-us/win...a-drive-letter