If I may, let's try NOT mixing things all together. (physical vs. logical).
If the flash drive is fully functional you do a dd-like or "forensic sound" PHYSICAL image (that is completely "agnostic" to the filesystem(s) used and to whether the filesystem(s) is/are valid or corrupted).
Then you make a copy (still "physical") of the image and either carve it for the files or attempt rewriting/fixing/repairing the filesystem structures.
Be warned that a chkdsk (even without "f" option) SHOULD (if it is the case) NEVER be run on the actual flash device directly if NOT through a writeblocker (only to underline the need of what athulin posted).
BUT, *any* disk imaging tool will work fine with a fully functional device, so, as already said before, it is very likely that you have a much "deeper" problem, including unreadable (faulty) areas of the device.
Your best option, if this latter is the case, is actually ddrescue (or a similar "recovery oriented" tool) that will automate (given the correct options) the skipping of the sectors and also further attmpts to read.
jaclaz
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