Similar to a Bitcoin Core reset problem. All lost?

[DAC1993]

Greetings! All help is greatly appreciated.

I store Bitcoin Core wallet on an external hard drive. All the files related to the wallet live on the hard drive (as far as I know). Whenever I need for it to catch up on transactions I connect it to one of my computers (always same one) and it updates.

Sequence of events:

Bitcoin Core wallet lives on an external hard drive with no issues for a while.

For an unrelated reason I reinstall Windows on the computer. (Including this in the timeline, so you understand the computer is in a factory reset condition right now)

I connect the Bitcoin Core hard drive to that computer as usual and run the wallet. It starts running as brand new (inquires if the program is being run for the first time and offers to choose where to store the loaded data, starts to catch up from the beginning). No transactions reflected. No addresses stored. I don't know how to properly open the wallet.dat.

I run a Litecoin Core from same external hard drive. Also shows the window as if the program run for the first time. I set the directory to the same folder it used to be (within the litecoin wallet on the same external hard drive). Happy ending, wallet catches up, all funds there.

I haven't let the Bitcoin Core catch up yet. Should I? All advice greatly appreciated, I am still in the middle of my education.

Here is a case that I found to be closest to mine. Except there isn't a reason why my wallet.dat would be reset (because the wallet lived on an external hard drive, not connected to computer). https://bitcointalk.org/index.php?topic=1355500.0

Bitcoin Client Software and Version Number: Bitcoin Core 0.15.0.1
Operating System: Windows 10
System Hardware Specs:
Description of Problem: See below
Any Related Addresses:
Any Related Transaction IDs:
Screenshot of the problem: not available
Log Files from the Bitcoin Client:

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[achow101]

Did you choose to set the datadir to where it was originally on the external drive? If not, it is using the default location and it won't know to look for the data on your external drive.

[DAC1993]

achow101, Thank you for your response. I was able to restore the funds.

In a nutshell:

I let the Bitcoin Core catch up; this is why i am getting back to you after a while. The funds were not there.

There were 2 external hard drives with 2 identical copies of wallets (bitcoin, litecoin) and 1 flash drive with one of each wallet.dat file copies. 1 of hard drives was used as a wallet and connected to the same computer whenever needed to be in active use. The other one just layed as a copy. All of the copies on the hard drives and the flash drive were created at the same time and since then nothing has been done to them (therefore wallet.dat files did not need to be updated on any of the hard drives or the flash drive, all of them were just stored offline and the last date modified for all wallet.dat files was the same). Then the reinstallation of Windows on the computer happens. Then the wallets are connected back to the computer and start running like brand new, request to set directories for data storage/operation. The directories are set to the same as they always used to be. Ltc core catches up, btc core says its empty. Then this happens. The wallet.dat files for btc core on hard drive and the flash drive found to have 2 different sizes. The file on the flash drive is about 10% smaller. In my mind, that could mean that although the "date modified" is the same for the flash drive file as for the hard drive, perhaps a transaction was added to the hard drive (the bigger) wallet.dat on that day, but not to the flash drive one. It is not correct - no transaction matches the "date modified". I try pasting the smaller wallet.dat file from flash drive in place of larger one on hard drive. Run it. It's all there.

Could you please help me reason through this? While researching I saw other people asking about similar situations; i'd like to be able to help people like that in the future. For example how is it possible that a larger .dat file gives out an empty wallet, while a smaller one turns out to have it? Why did ltc catch up, but btc did not while the programs were stored on same hard drive and the wallet.dat files were not altered. If you feel like the questions are demanding i'd love to buy you a coffee, so feel free to drop a LTC address. Thank you.