Glad you found the problem. For future usage, I suggest you consider setting a different fixed drive letter assignment to each drive that you use. These drive letters should be chosen further through the alphabet to reduce the likelihood of ever clashing with another device which has been given the next available letter. What I mean is that at random, a given cellphone or USB memory or whatever, might be assigned (say) letter E: Then even if you had specified that a particular drive should always ask for E:, when that was unavailable ir might be given F: instead. But if you have told it to always ask for (say) P:, then there will never be anything else that will be blocking this. Similarly, another drive might be told to always ask for Q: and then these two will never come into conflict no matter in which order they may be connected or disconnected.
Always after setting a new drive letter, of course Lightroom Classic may need to be re-browsed to employ this new drive reference instead of the previous one. But once that drive letter becomes guaranteed to remain unchanging hereafter, this will be the last occasion for doing so.