Why can't LR see the DNG's?

i utterly confused lr.3.6

 

what doing every new set of images, wedding, setting new catalogue on off board hd.

 

i import images dng's & that's after first off loading them original nikon raw files (i don't trust either lr or myself right) onto hd.

 

so, have 2x copies.

one original non-pp raw files

one open in lr dng's work upon.

 

now, few days later, plug in hd pc , lo! file name off line or missing.

 

this happens more not, maybe 75% of work flow stymatised having work out what's happened or have start again on pp.

 

at point, trying both open .lrcat file either within lr / file /  open recent or directly file on offboard hd.

 

i can see dng's lr seems unable so.

 

grrrrrrrrrrrrrrrrrr!

 

what error making?

 

thanks

pat agonia wrote:

 

 

now, few days later, plug in hd pc , lo! file name off line or missing.

 

what os? windows or mac?

 

if windows, when plug in hd, drive letter same? if not, explain problem.

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