>>/425/
It stores the tags. You are not permanently fucked, but it might take a little work to get back up and running, and if you have a lot of 'my tags' (as opposed to just, say, the public tag repository), it may be trickier.
It may be simpler to do this one-on-one by email (hydrus.admin@gmail.com) or discord DMs (https://discord.gg/3H8UTpb), but we can also do it here.
My best first suggestion is to try to copy as much of client.mappings.db to the nice drive. If your OS is doing some of the move but sperging out and cancelling due to a bad hard drive sector, maybe a program like FreeFileSync will let you copy most of the file and still leave the truncated half-copy in the nice drive after the error. I am not sure if FFS will do that, but I think some copying program or command line call will be able to do the front half. You may have more luck cloning this half-copied db, and if that works you should be able to just boot and reset your PTR processing cache and be back up and running.
If you absolutely cannot copy any of client.mappings.db, we'll have to see if we can possibly create a barebones one and fool the client into thinking that is the correct one.